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The Correlation Between Struc- 
ture and Function in the De- 


velopment of the Special 
Senses of the White Rat. 


A DISSERTATION 


PRESENTED 
TO THE 


Facuutty oF Princeton UNIVERSITY 
IN CANDIDACY FOR THE DEGREE 


oF Doctor or PHILOSOPHY 


By 
H. H. LANE 


NORMAN 
UNIVERSITY OF OKLAHOMA 
1917 


Accepted by the Department of Biology, 
June, 1915. 


HARLOW 
OKLAHOMA 


tl i en 


CONTENTS 

Page 
I ree ewe 5 
ee InVesLieagions 2.2002 8 6 
a MIeLNOGS. LL 9 

A brief statement of the chief results obtained in this 
i ee en ae ee 13 
The nature and function of the neurofibrillae_________ 15 
Experimental and structural data—introductory_-_-_-_-_~_ 21 
Experimental data on the sense of touch_____________ 22 
’ Structural observations on the tactile apparatus______ 28 
Summary of results on the sense of touch___________-_ oe 
Experimental data on the sense of equilibrium_______~_ 35 
Structural observations on the organs of equilibrium__ 38 
Summary of results on equilibrium__________________ 42 
Experimental data on the sense of smell_____________ 44 
Structural observations on the organ of smell_______~_ 48 
ey Of results on smel]l_______________________ 51 
Experimental data on the sense of taste______________ 53 
Structural observations on the organs of taste________ 56 
mnery oO. results on taste___.___.._______.______ 58 
Experimental data on the sense of hearing___________ 59 
Structural observations on the organ of hearing______ 60 
mumeary of results on hearing.__________.________- 63 
Experimental data on the sense of sight_____________-_ 64 
Structural observations on the organ of sight_______~_ 64 
env oresuits On sight_..___.___._.._______________ 69 

The cause of development and of differentiation in the 
SEE 2 70 
Secn) connections in the rat._._.._..__._...._...___-_ 75 
a 81 
el A Se 


IN 86 


a 


Oklahoma University Studies No. 8. 


| The Correlation Between Structure and 
Function in the Development of the 
Special Senses of the White Rat. 


H. H. Lane 


Two periods in the ontogenetic development of 
an organism were recognized more than thirty 
years ago by Wilhelm Roux, the first that during 
which the organs are formed, and the second that 
of the development of function in the organs pre- 
viously laid down. Within the organism as a whole 
there is, of course, no sharp line of demarcation 
between these two periods, for different organs, 
or even systems, are formed and become functional 
at different stages in ontogeny; or the two periods 
may overlap in the development of the same organ, 
as Harrison has shown to be the case with muscle 
fibers. The order of appearance of the organs in 
an embryo is also of significance, since it sometimes 
happens that the presence and functional activity 
of one is a necessary precursor to the formation of 
another organ. 

The immediate problem of this investigation 
is the determination of the nature and amount of 
the correlation between structure and function in 
| the development of the special senses in the white 
| rat during both prenatal and early postnatal life. 
| The work has been done in the Biological Labora- 

tory of Princeton University. I wish to extend my 
sincerest thanks for the many courtesies and facili- 
ties afforded me, as well as for many helpful sug- 
| gestions and criticisms, to Professor E. G. Conklin, 
| by whom the problem was suggested to me; to 


6 H. H. LANE 


Doctor Stewart Paton, a pioneer investigator in 
neurobiology, whose rich experience and knowledge 
of the field has been constantly at my service; and 
to Professor C. F. W. McClure, whose deep 
acquaintance with the facts of comparative anat- 
omy has made it possible for him to offer many 
valuable suggestions in the course of the investiga- 
tion. Also I acknowledge my great indebtedness 
to Professor L. W. Cole, of the University of Colo- 
rado, for very material assistance by way of cita- 
tions to the literature; and finally I am under many 
obligations to the State University of Oklahoma 
for a year’s leave of absence to pursue this investi- 
gation. 

The method of attack adopted may be stated 
in general terms as follows: 

To determine by physiological experimentation 
just when the embryo or young rat first becomes 
possessed of the senses of touch, taste, smell, equili- 
brium, hearing and sight, and by a histological 
examination of the nervous system, both central 
and peripheral, and of the sense-organs, to discover 
the structural development exhibited by the parts 
concerned in each case at the time when the func- 
tion is first apparent. 


Previous Investigations. 


No previous investigation along exactly paral- 
lel lines is known to me, and only a very few work- 
ers have concerned themselves with allied problems 
and methods of attack; and they have for the most 
part dealt with the lower vertebrates. An extended 
review of their papers can therefore be dispensed 
with here. 

WINTREBERT (’04, ’05) has published a num- 
ber of short papers recording the results of his 
experiments and observations upon a few species 


CORRELATION OF STRUCTURE AND FUNCTION 7 


of batrachians, notably the frog and the axo- 
lotl Working on very young embryos of Rana 
esculenta, for instance, at the time when the 
tail bud had just made its appearance and when 
the myotomes of the anterior part only of the 
trunk had become contractile, he made a transverse 
incision just caudad to the contractile myotomes, 
of such a depth as to transect the neural tube, the 
notochord, and a considerable portion of the endo- 
dermal tissue. Under these conditions he found 
that, within a few minutes after the operation, a 
simple pricking of the end of the tail with a needle 
results in an immediate contraction of the trunk 
‘anterior to the incision. The stimulus was trans- 
mitted only through the uninjured ectoderm of the 
ventral body wall. The power of reacting under 
the conditions of the experiment was present for a 
period of only four days in the ontogeny; after that 
the power was lost. He concludes, therefore, that 
there is a period of ‘“‘primitive sensitivity,’”’ charac- 
terized physiologically by its independence of mus- 
cular differentiation and of nervous connection 
between the motor plates and the neural tube. 
PATON (’07) undertook to determine the extent 
to which the heart beat and “the earliest responses 
to external stimulation .. . are 
eee event upon the functional gutiviy of a nervous 
system.” The forms studied ranged from amphi- 
oxus to Lacerta, though Pristiurus and Scyllium 
gave the clearest results. He found “that the func- 
tional activities of the body represented by the beat 
of the heart and the primitive movements of aband 
adduction of the body begin at a time when 
these phenomena may as yet neither be Naini 
as myogenic nor neurogenic in origin” 
“That general motility or reactions to wtiedult are 
initiated within the different organs, such as the 


8 H. H. LANE 


myotome or heart, and are at first autochthonous 
but later fall under the regulating influence of the 
nervous system.” . . . . “The appearance of 
neurofibrils may generally be considered to be an 
indication that physiological activity has already 
actually begun, or will soon begin in the tract in 
which they have been differentiated.’’ é 
“One of the chief histological characteristics of the 
fully differentiated nerve is that it contains neuro- 
fibrils, and every bit of evidence so far accumulated 
points to the appearance of these structures as 
marking the period of greatest physiological activity 
in any given nerve.” . . . . “It seems to be 
not at all improbable that impulses, centrifugal as 
well as centripetal in origin, may play an important 
part in the differentiation of the neurofibrils.”’ 
COGHILL (714) (and more recently HERRICK 
and COGHILL (715) ) has made a study of the 
reflex mechanism concerned in the production of 
the first swimming movements in the larva of 
Amblystoma. He finds that in the very young 
larva the Rohon-Beard cells are both extro-and 
proprio-ceptive elements of a very primitive, but 
complex, reflex are through which an extero-cep- 
tive stimulus passes cephalad on one side (the right, 
for instance) of the cord to commissural neurones 
near the posterior end of the medulla, and thence 
to the ventral horn cells of the opposite (left) side, 
from which in turn the motor impulse travels to 
the myotomes. By the contraction of the latter a 
proprio-ceptive stimulus is imparted to the Rohon- 
Beard cells of the left side of the cord that trans- 
mit the impulse by way of secondary neurones to 
the same commissure and thence to the ventral 
horn cells of the right side. These then produce a 
contraction of the myotomes with which they are 
connected. In this way, as the result of this unique 


CORRELATION OF STRUCTURE AND FUNCTION 9 


arrangement, alternate wave-like contractions of 
the myotomes on the two sides of the body are 
brought about resulting in swimming movements 
on the part of the embryo as a whole. 

SMALL (799) has studied experimentally the 
psychic development of the white rat, during a 
period extending from the first to the twenty-eighth 
day after birth. Several references to this paper 
will be found at various places in this paper. 

WATSON (’03) likewise has studied the devel- 
opment of the psychic faculties in the rat by means 
of a series of “standard problems,” and finds that 
psychic maturity is attained by the twenty-fourth 
day after birth. Correlated studies were made 
upon the establishment of medullation in both the 
peripheral and central nervous systems of this ani- 
mal, with the result of very conclusively disprov- 
ing Flechsig’s hypothesis. A further account of 
Watson’s results will be found in the appropriate 
sections of this paper. 

Some other papers germane to minor questions 
raised by my own observations will be considered 
in connection with the points to which they have 
relation. 


Material and Methods. 


The material used in this investigation con- 
sists of fifteen different stages in the development 
of the white rat, ranging from embryos with a crown- 
rump measurement of 714 mm. to young sixteen or 
seventeen days after birth, at which time all the 
special senses have attained functional activity. In 
the case of the prenatal stages the mother was first 
killed by severing the cervical cord by a quick cut 
with a pair of large bone forceps. The use of 
anesthetics was avoided for fear of possible delete- 
rious effect upon the embryos or fetuses. The 


10 H. H. LANE 


abdomen of the mother was then immediately 
opened by a median incision through the ventral 
body-wall, extending from the public symphysis as 
far cephalad as necessary. The uterus was removed 
and placed at once into a dish containing the nec- 
essary amount of a solution made up according to 
the following formula: 


Caleium. chloride: 2002302227 0.2 gram 
Potassium chloride: 2 ohne 0.2 gram 
poglum chioride. 2202 eae 9.0 gram 
Sodium bicarbonate___...._____ 0.1 gram 
Dextrose se ow ee ee 1.0 gram 
Distilled’ Waters) 020 ike une 1000.0 cc. 


The dish and its contents had previously been 
warmed to 38° C., and together with the uterus was 
transferred to a warm chamber where the embryos 
were removed and subjected to experiment. The 
temperature and humidity of the warm chamber 
were very nearly constant at all times, an effort 
being made to have conditions as favorable as pos- 
siblé for the success of the experiment. It was 
found that mammalian embryos are very suscept- 
ible to the shock of sudden changes in tempera- 
ture, frequently only a few moments’ exposure to 
room temperature sufficing to kill them. 

Careful records were made at the time of the 
details of the procedure in each experiment and 
of the results. For the most part the embryos were 
preserved in a solution of neutral formol made up 
as follows: The ordinary 40% formalin was neu- 
tralized or made slightly alkaline by an excess of 
magnesium carbonate. One part of this stock solu- 
tion was added to nine parts of tapwater, making 
a 4% solution of neutral formol. The embryos 
remained in this solution for not less than four 
days—in several cases much longer, even for sev- 
eral months. They were then subjected to the fol- 


CORRELATION OF STRUCTURE AND FUNCTION 11 


lowing procedure which is a slightly modified 
form of the Bielschowski-Paton (’07) method: 

1. Upon removal from the neutral formol the 
specimen is washed over night in running water, 
then rinsed three or four times with distilled water 
and put into a three-fourths of one per cent 
(0.75%) solution of AgNO, in the dark. In this 
it is left for a varying number of days, depending 
upon the room temperature, until it acquires a light 
brown color. 

2. The specimen is again rinsed in distilled 
water and put for two hours in the dark into a 
‘solution made according to the following formula: 


DS OES SS ee ee BU), Ce: 
es ee 4 drops 
Concentrated NH,OH_.~~-...--- 12 drops 


The addition of the NaOH to the silver solu- 
tion produces a dark brown precipitate which is 
dissolved by the NH,OH. In this solution the 
specimen becomes mahogany colored and more or 
less translucent. 

3. The specimen is again rinsed in distilled 
water and placed for fifteen minutes in the follow- 
ing solution to dissolve any connective tissue that 
may be present: 


penton |). Water 22 20 ce. 
Glacial Acetic Acid__._._._.._._ 10 drops 


In this it becomes yellowish brown in color. 

4. Again the specimen is rinsed in distilled 
water and put for twelve to twenty-four hours in 
the dark into a solution composed of: 


1% aqueous solution of hydrochinone_. 20 cc. 
Beemeurren formol. 0 a 2 ce. 


The time in this solution is determined by the 
size of the specimen. 


12 H. H. LANE 


5. Once more the specimen is rinsed in dis- 
distilled water, gradually dehydrated in _ alcohol, 
cleared in benzol or chloroform (not xylol), im- 
bedded in paraffin, and sectioned. Sections five to 
seven micra in thickness are best for most purposes. 
The sections are mounted on slides by the usual 
Mayer’s albumen fixative method and after being 
thoroughly dried are painted over with a 0.5% 
solution of celloidin, to prevent loss of sections in 
later processes. 

6. After the removal of the paraffin the sec- 
tions are passed down through the alcohols, rinsed 
in distilled water, and placed in the dark for two 
hours in a 0.1% solution of gold chloride neutral- 
ized with lithium carbonate. After the gold has 
been reduced and the sections have a dark grayish 
blue color, they are quickly rinsed in distilled water 
and then put for ten minutes in a 5% solution of 
sodium hyposulphite. 

7. The sections are now washed for two hours 
or longer in running water, passed up through the 
alcohols to absolute, where they are counterstained 
in a 1% solution of eosin in absolute alcohol, cleared 
in xylol, mounted in neutral balsam, and covered 
in the usual way. 

If the solutions are made up fresh as needed, 
all glass-ware kept perfectly clean, and if in every 
step of the process, except No. 3, care be taken to 
have all the fluids used neutral or slightly alkaline 
in reaction, uniformly good results may be expected 
by this method. 

One very satisfactory modification of this 
method consists in the omission of the gold chloride 
and subsequent treatment, i. e., steps No. 6 and 7. 
The sections when mounted on the slide were al- 
lowed to dry, the paraffin removed and the sections 
cleared in xylol, and covered with neutral balsam 


CORRELATION OF STRUCTURE AND FUNCTION 13 


and cover-glass in the usual way. The result is 
greater contrast between the nerve-fibers and the 
other tissues than that seen after treatment with 
gold chloride. In my own preparations, especially 
in the case of very long series, it was the practice to 
finish the odd-numbered slides by this method and 
and the even-numbered slides with the gold chloride. 
In addition to being in some respects more satisfac- 
tory for study, the slides finished by the shorter 
method require a much briefer time and less labor 


for their preparation. 


As a control method I have used the Ree oe: 
Huber (713) pyridine process, with decalcification in 
7% nitric acid. The published accounts of this tech- 
nique are so recent and so readily accessible that a 
description of it is unnecessary here. Its chief ad- 
vantages are the beautiful contrast between the 
nerve-fibers and the other tissues, and the fact that 
decalcification is possible, so that whole heads of 
young rats may be studied in serial sections. The 
chief disadvantages encountered in its use are, first, 
the long time required, and, second, the tendency of 
the brain tissues to swell when washing in distilled 
water after the pyridine. It has not been possible 
so to modify the Bielschowski-Paton method as to 
permit of decalcification. 


A Brief Statement of the Chief Results Obtained in 
This Investigation. 


A few points stand out prominently as the 
result of the investigation that is described in detail 
in the succeeding sections of this paper. 

1. The exceedingly early establishment of the 
general structural relationships between the nervous 
system and the other organs of the body. This 
comes at a time when the spatial relations are such 


14 : H. H. LANE 


as to make easy the proper extension of the sensory 
and motor nerves to their respective peripheral 
areas of distribution. 

2. Both the central and the neruhenn por- 
tions of the nervous system are laid down, at least 
in ground-plan, for a longer or shorter time before 
their functional activity begins. 

3. In the establishment of a sensory chain of 
neurones from the periphery to the center, the 
exteroceptive end-organ is the last link to be com- 
pleted. 

4. Immediately upon the completion of the 
structural development of the peripheral end-organ 
the definitive function of that Saad sensory 
chain is established. 

5. <A functional activity once established may 
be further perfected through the gradual addition 
of other neurones to those at first constituting the 
receptive path and the association paths within the 
brain. 

6. The gradual perfecting of the co-ordinating 
powers of the central nervous system which is a 
later development is to be explained in the same 
way. 

7. The whole nervous mechanism up to the 
point, at least, where the definitive functions first 
appear, develops not from the effects of extrinsic 
stimuli, but along predetermined lines as the result 
of inherent forces probably to be thought of as the 
product of the hereditary constitution of the fertilized 
egg. 

It is, of course, altogether probable that the 
normal course of events is influenced by many fac- 
tors which gradually and in succession enter into 
the situation to complicate matters, such as the 
establishment of the circulatory and lymphatic sys- 
tems, the excretory system, and probably some of 


CORRELATION OF STRUCTURE AND FUNCTION 15 


the ductless glands. But these are all factors inher- 
ent in the primary organization of the individual; 
they are therefore intrinsic factors in ontogeny. 
Extrinsic factors such as the stimuli of light, sound, 
or those of a chemical, electrical, or mechanical 
nature, etc., play little or no part in the establish- 
ment of the various functional activities of the 
nervous system, though they may later have an in- 
fluence during the time when co-ordination is being 
gradually perfected. 


The Nature and Function of the Neurofibrillae. 


In the following description of structural con- 
ditions there will be noted the assumption that the 
presence of neurofibrillae is an indication that the 
nerve or tract concerned is capable of functioning. 
It becomes necessary therefore at this point to dis- 
cuss the grounds for this assumption. 


Neurofibrillae may be demonstrated by means 
of any one of several technical methods including 
those of Apathy, Bethe, Cajal, Bielschowski, Don- 
aggio, Paton, Ranson and Huber. They are constant 
structures in that by proper means they can always 
be found in the neurones of all parts of the adult 
nervous system of the vertebrates and in many at 
least of the invertebrates. Nevertheless Auerbach 
has denied their existence and Pighieri considers 
them to be mere artifacts, the inconstant products 
of the precipitation of various substances by the 
reagents used in fixation. On the other hand, and 
with good reason, Apathy, Bethe, Cajal, and others, 
hold them to be normal constituents of the neurone. 
Hatai asserts that they constitute a reticulum lying 
in all parts of the neurone, the cross-meshes not 
ordinarily being seen, though he thinks himself able 
to demonstrate them by means of a special tech- 


16 H. H. LANE 


nique. In short it is the old controversy in special 
dress as to whether the structures visible in pro- 
toplasm that has been subjected to various fixing 
reagents are to be regarded as actually present as 
such in the living state or are the more or less 
altered and distorted products of such structures of 
the cell. Were our knowledge of the actual nature 
of living protoplasm more profound, possibly this 
particular form of the question would be answered. 
In the light of present-day results of cytological 
research it would appear that some structures seen 
under the microscope after the fixation of the tissue 
exhibit more of the effects of the fixatives than they 
do of the structure of the living protoplasm. It is 
highly probable that some of the granules and reti- 
cula so frequently seen and described are more or 
less the precipitation products of different constitu- 
ents of the living protoplasm; on the other hand it 
can be demonstrated that mitochondria, spindle- 
fibers, and chromosomes, at least, are actually pres- 
ent as such in the living state. In some cases 
however what appear to be granules in fixed mate- 
rial exist in life as isolated portions of the living 
colloidal gel having a different degree of viscosity 
from the surrounding substance. Thus the neurofi- 
brillae are either to be regarded as rows of such 
colloidal particles held more or less closely together 
in a linear arrangement by means of another con- 
stituent of the protoplasm differing from them in its 
degree of viscosity, or the fibrillae may consist en- 
tirely of such a viscid substance which has the form 
of strands differing chemically and physically from 
the other elements of the surrounding protoplasm. 

If these considerations hold true, then the view 
of Koltzoff, upheld for the neurones by Gold- 
schmidt, Sztits, and others, namely, that the form 
of any cell is determined by the shape of a solid 


CORRELATION OF STRUCTURE AND FUNCTION 17 


framework within it, must be materially modified. 
The so-called ‘“Stiitzgeriist’”’ of the cell is not an 
unyielding structure like the bony skeleton of a 
mammal or the steel-frame of a skyscraper, but 
rather a meshwork of a substance a little more 
viscid than some other portions of the protoplasm. 
The “skeletal theory” of cell structure proposed by 
Koltzoff seems to rely for its proof upon the analogy 
of the action of liquid masses when in contact with 
solid bodies. Thus Plateau has shown that lquid 
masses conform in their shape to that of the solid 
body with which they may be in contact. 

The idea that in a neurone the neurofibrillae 
constitute this “Stiitzgeriist” has been advanced by 
Goldschmidt and others, but it has met with some 
serious objections. Thus Marinesco (’15) in a recent 
paper argues very soundly that (1) Koltzoff and 
Goldschmidt have not shown conclusively that the 
neuronal cytoplasm is a fluid, which it should be 
on the basis of the analogy just mentioned. (2) 
Brownian movements are not exhibited by the col- 
loidal particles found in the cytoplasm of the neu- 
rone until the viscosity of the hyaloplasm has been 
reduced, as has been shown experimentally. (8) On 
the basis of his own observations, Marinesco main- 
tains that the hyaloplasm and the neurofibrils are 
both more or less fluid gels which differ only in the 
degree of their viscosity, the neurofibrils being the 
more stable. (4) The tearing away of a spinal or 
cranial nerve usually results in the total destruction 
of the neurofibrillar structures; yet, on the other 
hand tumefaction followed by an atrophy of the 
neurone does not lead to such profound modifica- 
tions of the cellular form as the theory of Koltzoff 
and Goldschmidt would seem to demand. (5) The 
destruction of the neurofibrils that follows shortly 
after the death of an animal does not result in the 


18 H. H. LANE 


collapse of the cell-body of the neurone. (6) The 
neurones of animals that have undergone hiberna- 
tion or freezing exhibit marked changes in their 
neurofibrillae, so that the latter can hardly be con- 
sidered to possess such a permanent character as 
the theory demands. In fact such observations have 
served to establish the fact that the neurofibrils 
undergo continual change. (7) After transsection of 
nerve trunk, the neurofibrils peripheral to the sec- 
tion undergo regressive modifications that end ulti- 
mately in their complete destruction, yet the axis 
cylinder as a whole does not crumble, collapse, or 
otherwise fall into such a dissolution as it should 
exhibit if the neurofibrils play only a mechanical 
role in the support of the hyaloplasm. (8) In va- 
rious pathological states more or less extensive 
lesions occur in the neurofibrillar reticulum without 
and corresponding modifications of the cellular 
form. 

It would appear therefore that the neurofibrils 
cannot be regarded as the ‘‘Stiitzgeriist’’ of the 
neurone. What then is their function? 

This is a question much more easily raised than 
answered in the present state of our knowledge. 
Apathy, Bethe, Paton, and others, hold the view 
that they function in the conduction of the nervous 
impulse. It is at least questionable whether the 
evidence which they advance is conclusive. Indeed, 
it seems probable that too much reliance has been 
placed upon the analogy so frequently made be- 
tween the nervous system and a telegraph system 
in which the nerves and the nerve fibers of the 
former correspond to the cables of the latter, and 
the neurofibrillae to the individual wires. The ex- 
periments of Ducceschi and Bethe on the effect of 
the compression of nerve-fibers do not appear suf- 
ficiently conclusive. 


CORRELATION OF STRUCTURE AND FUNCTION 19 


A clue to the true conditions may be furnished 
by the fact that the relative proportions in the 
amount of the fibrillae and of the perifibrillar sub- 
stance differ in medullated and in non-medullated 
nerve-fibers. It is well established that medullated 
fibers are better conductors than the non-medul- 
lated. This has generally been ascribed to an in- 
sulating power on the part of the myelin sheath 
though recent work would appear to render it prob- 
able that the latter may serve rather a_ trophic 
function. The fact that medullation does not begin 
anywhere within the central nervous system of the 
white rat until several days after birth leads to the 
suspicion that possibly the importance of medulla- 
tion from the standpoint of insulation may have 
been overestimated in the past. On the other hand, 
the better conducting-power of the medullated fibers 
may be ascribed to the fact that they are made up 
of relatively a larger proportion of neurofibrils than 
of perifibrillar substance, the latter in fact being 
quite inconspicuous, while in the so-called non- 
medullated fibers the reverse is the case. In the 
latter type of fibers, the fibrillae form a small and 
inconspicuous core with a very thick sheath of peri- 
fibrillar substance surrounding them. It is evident 
that since the neurofibrillae and the perifibrillar 
substance are the only parts of the nerve-fiber that 
have a continuous distribution throughout the whole 
extent of the fiber, one or the other must be the 
conducting element. Where there is a variation in 
their relative amounts, therefore, the better power 
of conduction will lie with that fiber which has the 
greatest amount of the conducting element. Since 
then the medullated fiber is the better conductor 
and at the same time has a relatively greater quan- 
tity of neurofibrillae than of perifibrillar material, 


20 H. H. LANE 


as compared with the reverse condition in the non- 
medullated fiber, the conclusion is inevitable that 
the neurofibrillae, and not the perifibrillar sub- 
stance, constitute. the conducting element of the 
neurone. Since the perifibrillar substance certainly 
is just as much a continuous layer in the transverse 
direction as in the longitudinal one, and since fur- 
thermore the same is not true of the fibrillae, they 
being continuous only in the longitudinal direction, 
the fact, which has been demonstrated experimen- 
tally, namely, that an electric current is not trans- 
mitted across a nerve but only in a longitudinal 
direction, adds to the probability of the correctness 
of our conclusion. Furthermore it may be argued 
that in the so-called non-medullated fibers it is the 
perifibrillar substance that furnishes the needed in- 
sulation for the fibrillae, while in the so-called med- 
ullated fibers the lack of perifibrillar substance is 
compensated by the addition of the myelin sheath. 
To these considerations may be added the further 
argument which follows from the results of Paton’s 
work on Pristiwrus and others of the lower verte- 
brates, namely, that the functional activity of any 
part of the nervous system is never fully established 
until after the completion of fibrillation. 


If therefore an absolutely conclusive answer 
cannot now be given to the question of the function 
of the neurofibrillae, it is most probable, in the light 
of our present knowledge in this field, that the power 
of conducting nerve-impulses lies rather in the neu- 
rofibrillae than elsewhere. At any rate, the pres- 
ence of neurofibrillae may be taken as an indicator, 
a criterion of the functional state of the neurone, 
and as such it will be used in the description of 
structural conditions given below. 


CORRELATION OF STRUCTURE AND FUNCTION 21 


Experimental and Structural Data. 


While the experiments described below show 
that there is a certain amount of overlapping of the 
periods when the various senses make their appear- 
ance, still in a general way it may be said that the 
order in which they attain functional activity is as 
follows: 


Touch. 
Equilibrium. 
Smell. 
Taste. 
Hearing. 
Sight. 


It will be seen that certain minor inconsist- 
encies appear in the record of the experiments on 
successive days. This is due to the fact that when 
hungry or after a period of rest the animals re- 
spond to certain stimuli more readily than they do 
after a full meal or when fatigued. In general, 
however, it is clear that after the first indication of 
functional activity on the part of any of the special 
senses, succeeding days reveal a gradual perfecting 
of the animal’s powers. This is undoubtedly to be 
ascribed to a gradual increase in the perfection of 
the association centers, as well as to the addition of 
an increasing number of exteroceptive end-organs. 
For example, the sense of touch is at first mani- 
fested most clearly in the snout region at a time 
when there are present the anlagen of only a dozen 
or so vibrissae on either side. As development pro- 
ceeds other vibrissae are added to those first present 
and the ordinary body hairs acquire a sensory in- 
nervation, as does also the integument generally. 


D> OU ye 8 DO 


22 H. H. LANE 


EXPERIMENTS ON THE SENSE OF TOUCH. 


The 714 mm. embryos gave no evidence of hav- 
ing a sense of touch, although they were stimulated 
with a fine sable brush, and gently pricked with 
the point of a fine needle on various parts of the 
body, the limb-buds, and the head. Electrical stimu- 
lation with an induction-coil produced no apparent 
reaction except a variation in the rate of the heart- 
beat. 

By the time the embryos had reached a length 
of 16 mm. (crown-rump measurement) slight but 
readily perceptible movements of the body were 
noted upon pricking with a fine-pointed needle. 
These were most marked when the stimulus was ap- 
plied about the flanks and sides of the body and 
the snout. Stimulation with a fine sable brush 
failed to evoke any response. That the response to 
the needle-prick on the snout was due to nerve- 
innervation and not to direct stimulation of a motor 
mechanism is shown conclusively (1) by the fact 
that the movement called forth involved the turning 
of the head as a whole, and (2) by the additional 
fact that the sections show no sign as yet of the 
histogenesis of muscle in the snout region. Further- 
more the reaction was too promptly made to permit 
of the stimulus being transmitted through the gen- 
eral protoplasm. 

In embryos of 23 to 28 mm., crown-rump 
length, stimulation with the brush, as well as gentle 
pricking with the needle-point, about the shoulder, 
upper arm, hip, rump, and thigh, resulted in move- 
ments of the limbs or body-wall as the case might 
be. Stimulation of the vibrissal region of the snout 


CORRELATION OF STRUCTURE AND FUNCTION 23 


produced movements of the head decidedly more 
vigorous than before recorded. 

Fetuses 3.5 cm. in length from tip of snout to 
base of tail were very active, squirming and kick- 
ing about while yet within the uterus. Upon removal 
they were found to be very sensitive to stimulation 
with the brush, needle-prick, and induction-current, 
fully as much so, apparently, as newly-born young. 
They responded to stimulation on the flanks, sides 
of body, front and hind-limbs, toes, tail, neck and 
head, by more or less violent wriggings and twist- 
ings of the body, movements of the limbs, spreading 
of the toes, etc. One hour after removal from the 
uterus, they responded with faint squeaks upon 
stimulation with the needle-prick, undoubtedly show- 
ing the presence of pain-sensation. Gentle stimula- 
tion of the flank with a sable brush caused the 
body to be bent laterad into the form of a C, that 
is, the head and posterior end of the body were 
turned toward the side stimulated. Upon a pro- 
longation of the stimulus, the anterior and posterior 
extremities of the body were jerked back to or 
slightly beyond a straight line corresponding to the 
longitudinal axis of the body. When the stimula- 
tion was still further prolonged, writhing and jerk- 
ing movements were made that persisted for a few 
seconds after cessation of the stimulation. Milder 
stimulation, as with a single hair, called forth little 
or no response except when applied to the snout, 
the region of the vibrissae being decidedly more 
sensitive than other regions of the head or body. 

When young rats (4.3 cm. long) only a few 
hours old were gently stimulated by touching the 
sides of the body with a sable brush, they responded 
by contortions of the body, movements of the limbs, 
both fore and hind, and gave vent to audible 
squeaks. But here again it was found that the 


24 H. H. LANE 


region of the vibrissae was especially sensitive. 
SMALL (799) notes for this stage (the earliest 
heretofore examined) in the rats which he had 
under observation that ‘“‘they give little response to 
light pressure, as with a hair,—except upon the 
nose, which seems to be very sensitive. Mass pres- 
sure is not noticed unless comparatively strong.” 

Rats thirty to thirty-six hours old when stimu- 
lated with a small sable brush were found to be 
very sensitive about the vibrissae, flanks, and mid- 
dorsal line of the body. When stimulated on the 
flanks, some (the larger and more vigorous indi- 
viduals) responded quickly with an attempt to push 
away the brush with the hind-foot of the same 
side, the toes being spread well apart; others 
(smaller in size but belonging to the same litter) 
made a less vigorous response, more apparent when 
the brush was applied to the ventral part of the 
flank. Apparently voluntary (?) scratching move- 
ments with the hind foot were noted at times in the 
larger individuals. There were distinct reactions 
to the needle-prick on the foot, shin, thigh, tail, 
hand, fore-arm, upper arm, shoulder, sides, flanks, 
top and sides of the head, cheeks, and region of the 
vibrissae. Electrical stimulation with an induction- 
coil applied to the back of the head, along the en- 
tire length of the spinal column, of the trunk and 
tail, the legs, sides of the body and belly, all re- 
sulted in decided reactions, the movements amount- 
ing to contortions in many cases. Stimulation of the 
feet in the same way produced a spreading of the 
toes toa slight but appreciable extent. 

In the case of 55-hour-old rats, stimulation 
with a sable brush resulted in attempts to remove 
the irritating object by kicking and scratching move- 
ments of the hind legs and feet. They were most 
sensitive on the sides of the body and flanks, though 


CORRELATION OF STRUCTURE AND FUNCTION 25 


nearly as much so on the limbs. Stimulation of the 
top of the head and back produced the same re- 
sponses but only after the lapse of several seconds, 
that is, after prolonged stimulation. Touching the 
region of the vibrissae resulted in twitching move- 
ments of the upper lips. Stimulation of the tail 
resulted in its being tucked in underneath the body 
and between the hind legs. Brushing the median 
side of the hind foot resulted usually in violent con- 
tortions of the hind quarters together with the 
drawing up of the hind feet along the sides of the 
body; frequently when the left hind foot was stimu- 
lated with the brush, the right one was used to 
scratch the right flank, or vice versa; a few seconds 
later the stimulated foot was also drawn up in the 
same way to scratch the flank on its side. The 
median side of the foot seemed to be more sensitive 
than the outer side. 

Gentle pricking with the needle on the frontal, 
occipital and parietal regions of the head resulted 
in immediate response; the whole body was vio- 
lently contorted and movements of the hind feet as 
though to brush off the irritation were noted. Prick- 
ing of the lower jaw or snout resulted in violent 
attempts to wipe off the irritating object with the 
fore paws, usually both paws being used simul- 
taneously, and even overlapping each other, the 
one on the side stimulated being underneath the 
other. These movements were followed after repe- 
tition of the stimulus by an opening of the mouth, 
extension of the tongue, and movements as of swal- 
lowing. When the hind feet were in such a position 
that they could not be well used, the front feet 
were employed to wipe off the irritation on the 
back of the head. The tail, feet, legs, and fore- 
quarters were very sensitive to needle-pricks; the 


26 H. H. LANE 


hind-quarters less so. Reactions to electric stimuli 
were violent. 

Small’s results on this stage were in part mark- 
edly different from those recorded here. He says: 
“Irritating fluid (HCl) produced instantaneous re- 
sponses from all. In addition to the motor reactions, 
there were vocal expressions and a striking acceler- 
ation of respiration. Reactions to the other stimuli 
were slow, varying from ten to fifty seconds.” 

In the case of three-day-old rats the skin 
seems not so sensitive as in earlier stages. Upon 
stimulation with sable brush on hind-quarters, hind- 
legs, flanks, sides of body, and back, no noticeable 
response was called forth. Brushing the shoulders 
and fore-limbs, sometimes, but not always, occa- 
sioned a movement of the hind-limbs as though to 
scratch or push off the irritating object. The same 
sort of stimulation applied to the top and sides of 
the head produced no apparent response. Stimula- 
tion, with the brush, of the snout and the region of 
the vibrissae, if prolonged, produced squeaks and 
movements of the head as though to avoid the irri- 
tating object, but no movements of the forelimbs 
and paws. 

Gentle pricking with a needle of the rump, 
thighs, hind legs and tail produced no appreciable 
response. Pricking of the hind feet resulted in a 
violent attempt to tuck them under the body. Prick- 
ing along the vertebral column from the lumbar 
region cephalad, over the head, sides of the body 
(but not the flanks), fore-limbs and paws resulted 
in violent contortions of the whole body, rather 
than in any specific response of the parts directly 
stimulated. Response to electrical stimulation was 
not so violent as in earlier stages. 

With five-day-old rats practically identical re- 
actions were obtained. Small’s results, again, are 


CORRELATION OF STRUCTURE AND FUNCTION 27 


not wholly in accord with those recorded here. He 
notes for the 5th to 8th days inclusively that the 
“Dermal sensitivity becomes more acute, though 
susceptibility to pressure is still greater on the nose 
than elsewhere on the body. Especially, greater 
when tickling is involved. A bristle drawn across 
the body elicits scarcely any response; but applied 
with the same pressure to the nose, it evokes 
squeaking and vigorous head-shaking”’ (on the 7th 
day). “When the toes are touched the rats squeak 
and jump so as to lift the body nearly off the floor. 
One, thus insulted, crawled away two inches.”’ 

Rats nine days old were very sensitive to 
touch all over the body, legs, and head, responding 
not only by muscular movements but also by 
squeaks. 

When gently pinched on the cheeks and sides 
of the heads in front of the ears they sought to 
push away the offending object with their fore- 
paws. When pinched gently on the top or back of 
the head the hind foot of the same side was brought 
forward to push away the forceps; the same reac- 
tion was evoked when the sides and flanks were 
gently pinched. Pinching of the toes sometimes 
produced an instant response—squeaking and re- 
traction of the legs—but sometimes the response 
was very slow or even absent. The pinna of the ear 
was very sensitive to touch. Pinching the tail, rump, 
etc., resulted in squeaking and turning movements 
of the whole animal—sometimes it whirled end-for- 
end almost instantly. 

At an age of twelve days there was not mani- 
fested such sensitiveness to light pressure, e. g., of a 
brush, as at previous times. A needle prick, except 
about the base of the vibrissae, must be accom- 
panied by considerable pressure to evoke a marked 
response. Pricking about the region of the vibrissae 


28 H. H. LANE 


resulted in violent responses and vigorous rubbing 
of the region on both sides of the head with both 
fore-paws. Small notes that the ‘‘Dermal sensitiv- 
ity [is] considerably heightened. One jumped vio- 
lently when touched with the sharp corner of a 
piece of paper. Flanks, sides, back and feet are 
equally sensitive.”’ 

In rats sixteen days old the vibrissae were very 
long and in constant use. When pinched about the 
face with fine forceps, one grabbed them with its 
jaws and bit them forcibly enough to make a dis- 
tinctly audible gritting sound. Otherwise this stage 
revealed practically the same conditions as are re- 
corded for the preceding stage. 


Structural Observations on the Tactile Apparatus. 


In the 7144 mm. embryos a large number of as- 
sociation fibers are already present in the cord, and 
brain stem. The anlagen of the vibrissae are not 
yet apparent. The innervation in the snout region 
comprises two branches of the fifth nerve. Of these 
the ramus ophthalmicus profundus trigemini has 
the form of a small bundle of fibers, tapering off to 
a single fiber at the distal end, deep within the 
mesenchyme, connection with the more superficial 
tissue not yet having been established. The ramus 
maxillaris trigemini extends slightly anterior to the 
optic cup, but ends before reaching the surface 
ectoderm. Sensory fibers of the spinal nerves do 
not extend to the ‘periphery. 

In the 16 mm. embryos, about a dozen anlagen 
of vibrissae are present on each side of the snout. 
The vibrissae themselves do not extend to the sur- 
face. The ramus maxillaris trigemini in the form 
of a large trunk with many fibers runs to the snout 
region where it breaks up into a “brush” by the 


CORRELATION OF STRUCTURE AND FUNCTION 29 


spreading apart of its branches until finally some of 
the fibers end in a “basket” or reticulum in the 
follicles around the bases of the vibrissae. The 
ramus mandibularis trigemini has a similar distribu- 
tion to the vibrissae on the lower jaw. Proximad 
the maxillaris enters the Gasserian ganglion through 
which none of its fibers can be individually traced, 
although it is possible to find many of them in con- 
nection with ganglion cells. In other words, the 
distal fibers of The trigeminus, so far as can be 
determined, are axones of neurones located in the 
Gasserian ganglion. The ganglion itself is con- 
nected with the anterolateral margin of the myel- 
encephalon by a large trunk of fibers which run 
dorsad along the anterolateral face of the hind- 
brain for a considerable distance, then turn sharply 
caudad to enter the medulla in which they join or 
constitute a large ventro-lateral tract. 

In the 23 mm. embryo the number of anlagen 
of vibrissae has increased to more than thirty on each 
side of the snout. Those present in the earlier stage 
described above are much farther advanced in their 
development than those whose appearance is more 
recent. In the former the vibrissa itself is distinct 
from the follicle and the usual structural character- 
istics of both are shown. The relatively simple 
“basket” of fibers noted in the follicles in the 16 
mm. stage is represented now by a much more com- 
plex felted layer of fibers buried between two lay- 
ers of the follicular cells and forming a fibrous 
lamina about equal in thickness to the follicular 
layer between it and the root of the vibrissa. 
Whereas in the 16 mm. stage only one or at most 
a very few fibers were distributed to each vibrissa, 
in the 23 mm. embryo each of the older vibrissae is 
innervated by a large number of fibers forming a 
well defined branch of the ramus mazxillaris tri- 


30 H. H. LANE 


gemini. Moreover, the general ectoderm covering 
the snout has extending toward it a few branches of 
the same nerve. These fibers do not exist in suf- 
ficient numbers to constitute branches anywhere 
near the size of those innervating the vibrissae, and 
they follow in nearly every instance a course par- 
allel to a small blood-vessel running toward the 
surface between the rows of vibrissae. So far as 
could be determined these cutaneous nerve fibers 
have not yet come into contact with the ectodermal 
layer covering the snout. 

The motor branch of the trigeminus can be 
distinguished from the sensory branch at this time; 
it is completely fibrillated and can be traced proxi- 
mad into the medulla in which it runs slightly pos- 
teriorly in a ventro-dorsal direction to its nucleus. 
The latter is well defined; apparently it has no 
correlation tracts connecting it with any other part 
of the brain. The proximal connections of the sen- 
sory bundles of the trigeminus are the same as 
noted in the 16 mm. embryo. They enter the 
medulla just posterior to the motor branch and turn 
sharply caudad in the ventro-lateral marginal velum 
of the hind-brain. No correlation tract between the 
trigeminus and the corpora quadrigemina can be 
detected. All portions of the trigeminus seem to 
be completely fibrillated. Correlated with the in- 
creased size of the trigeminal trunks there is a 
marked increase in the number of the neurones in 
the Gasserian ganglion from which fibrillated proc- 
esses can be seen extending either proximad, or 
distad, or both. There is also noticeable a tendency 
for the fibers which extend through the ganglion 
for any considerable distance to be arranged in 
definite tracts which lie nearly parallel with each 
other and to the long axis of the ganglion itself. 


CORRELATION OF STRUCTURE AND FUNCTION 31 


The chief advance noted in 26 to 28 mm. em- 
bryos in connection with the trigeminal system, 
aside from a continuation of the lines of develop- 
ment just described for the 23 mm. stage, consists 
in the appearance of numerous fibers passing be- 
tween the medulla and the corpora quadrigemina; 
that is, co-ordination between these two parts of 
the brain is certainly possible now, if not earlier. 
These correlational fibers are to a certain extent 
grouped into a great number of small bundles each 
with only ten to twenty, or possibly in some cases 
more, fibres. Consequently it is manifestly impos- 
sible to identify by name at this stage the tracts 
that will be present in this region in the adult. 

In the 3.5 cm. fetuses the motor branch of the 
trigeminus is much larger than in the preceding 
stages, but runs in the same way to its nucleus of 
origin in the medulla. This nucleus likewise is 
larger in extent than before, though its cells are 
not apparently more numerous. They are, however, 
more widely separted from one another and in the 
spaces between them numerous correlation fibers 
from the lower levels of the medulla pass cephalad 
to the midbrain, or vice versa. The sensory fibers of 
the trigeminus are likewise more numerous than in 
the preceding stages. Many of them upon entering 
the medulla follow the same course as that already 
described for them in younger embryos; others en- 
ter more deeply into the substance of the medulla, 
some in fact almost reaching the floor of the fourth 
ventricle. Anteriorly a well defined portion of the 
sensory root fibers of the trigeminus pass in a dorsal 
direction through the antero-ventral portion of the 
medulla to the posterior corpora quadrigemina, and 
there are indications, though rather slight as yet, 
of a portion of this tract passing on into the an- 
terior corpora quadrigemina. 


32 H. H. LANE 


The snout region of the newly born rat not 
only shows a greatly increased number of the vibris- 
sae, but the anlagen of the ordinary body hairs are 
almost innumerable and the dermis contains a rich 
plexus of nerves which also extends through the 
stratum germinativum into the stratum intermedium 
of the epidermis. This is, of course, the usual type 
of ending for the organs of touch and general sensi- 
tivity, and those functions are therefore completely 
provided for at birth in the white rat. The fibrillar 
basket in the follicles of vibrissae can be seen to 
have the form of elongated cylinders at the base of 
which a bundle of nerve fibers leave the follicle at 
some distance, however, distad to the base of the 
latter structure, which extends deeply into the der- 
mis. The distal ends of the various trigeminal 
branches are composed of a greater number of 
fibers than heretofore, and of course the same is 
true also of the main trunks of this nerve. Of its 
central connections nothing new can be said, and 
aside from an increase in the number of fibers in 
the tracts leading to the cerebellum and to the 
corpora quadrigemina, a condition well marked by 
the ninth day after birth, no further appreciable 
advances are made in the structure of the tactile 
system during the time under consideration in this 
paper. 

In general, meaning by that to include other 
forms as well as the rat, the relations of the sensory 
roots of the trigeminus with the cortex are at best 
little understood. According to Edinger, “‘The cor- 
tical area and the central path of the sensory por- 
tion of the nervus trigeminus from the cortex to the 
capsula are yet unknown. Following pathological 
experiences, its fibers must lie in the posterior third 
of the capsule. The cortical tract of the trigeminal 
ends, in rabbits at least, in the ventral portion of 


CORRELATION OF STRUCTURE AND FUNCTION 83 


the thalamus. Leading up to it is a large bundle 
from the opposite nucleus of the bulb. And in this 
nucleus itself terminate the processes from the cells 
of the Gasserian ganglion. The ascending root con- 
tains the tactile nerves of the face as is shown by 
pathology.” 


Summary of Results on the Sense of Touch. 


1. 744mm. embryos: 


a. 


b. 


This stage, the youngest examined, gives no evi- 
dence of the possession of the sense of touch. 


A large number of correlation or coordination 
fibers are already present in the cord and brain 
stem. Both sensory and motor fibers of the spinal 
nerves are present though the former do not reach 
the periphery. The snout region is innervated by 
two branches of the trigeminus nerve, which, 
however, end within the mesenchyme i. e., do not 
reach the periphery. No anlagen of vibrissae 
are apparent. 


2. 16mm. embryos: 


a. 


The tactile sense is present on the flanks and 
snout as evidenced by motor responses to needle 
pricks. There is no response to stimulation with 
a sable brush. 


About a dozen anlagen of vibrissae are found on 
each side of the snout; these are innervated by 
branches of the maxillary division of the tri- 
geminus, which end in a basket-like reticulum in 
the vibrissal follicle. 


3. 23 to 28mm. embryos: 


a. 


b. 


They respond to stimulation with a fine sable 
brush as well as with a needle-prick; the snout 
region is most sensitive, though stimulation 
about the shoulder, upper arm, hip, rump, and 
thigh, also evokes motor responses. 


There is a noticeable increase in the number of 
the vibrissae as well as greater complexity in the 
neurofibral basket in each vibrissal follicle. The 


34 


H. H. LANE 


number of neuro-fibers of the trigeminus inner- 
vating the vibrissae is greatly increased. The 
general integument of the snout region has not 
yet received the terminations of other branches of 
the trigeminus, though many such are extending 
toward it, for the most part parallelling blood 
vessels in their course. In the 23-26 mm. em- 
bryos association paths exist between the medulla 
and the mid-brain. 


4. 3.5 cm. fetus and new-born rats: 


a. 


The tactile sense is still better developed over 
practically the whole of the body, tail and limbs. 
The snout is the most sensitive as shown by 
response to stimulation with a single hair. Pain 
or discomfort is now shown by squeaks. 


There is an increased number of vibrissae on the 
snout; the anlagen of ordinary body-hairs are very 
numerous, and the integument contains a rich 
plexus of nerve-fibers extending (in the snout) 
through the stratum germinativum into the strat- 
um intermedium. There is an increased number 
of sensory fibers of the trigeminus ending in the 
snout region. The central connections are better 
marked and more extensive than in the preceding 
stages. The fibrillar baskets in the vibrissal fol- 
licles are now elongated, felted cylinders, from 
the base of which the neurofibers in a relatively 
large bundle emerge some distance distad to the 
base of the follicle itself. 


5. Older stages: 


a. 


Throughout the older stages examined there is in 
general no particular advance in tactile sensi- 
bility over that just described. There is a con- 
tinued superiority of the snout region over the rest 
of the surface in sensitiveness to tactile stimuli 
and the use of the vibrissae as “feelers” is more 
and more marked. 


The structural advance in the tactile apparatus 
during these later stages is confined to an increase 
in the perfection of the mechanism already de- 
scribed. 


CORRELATION OF STRUCTURE AND FUNCTION 35 


EXPERIMENTS ON THE SENSE OF 
EQUILIBRIUM. 


The earliest indications of a sense of equili- 
brium were observed in the case of the 3.5 cm. 
fetuses. One hour after their removal from the 
mothers’ uterus they were able to sit upright on the 
belly with the forepaws placed well apart and the 
head up. At intervals the head was raised and 
moved from side to side, then returned to a resting 
position with the ‘‘chin” on the bottom of the dish 
or on one fore-leg. They were able to regain this 
position after stimulation with a brush applied to 
the flank had caused them to bend the body laterad 
into the form of a C, and upon prolongation of the 
stimulus writhing and jerking movements of a some- 
what violent character had followed. Turning them 
over on their backs did not result in attempts to 
right themselves, except very rarely, and then the 
efforts were very feeble. 

Young rats nine to ten hours after birth 
crawled awkwardly about over one another, and 
nosed about in an evident attempt to find the moth- 
er’s nipples. Without artificial stimulation they 
would roll over onto the belly, sides, or back at 
will; turned the head from side to side; kept their 
tails tucked beneath the body between the hind 
Iegs. When turned over on their backs by the ex- 
perimenter they made awkward righting move- 
ments, which sometimes succeeded. They had much 
better use of their forequarters than of the hind; 
they could spread their front legs apart so as to 
support the head in an upright position. At eleven 
to twelve hours of age they were able to crawl over 
the edge of a Petri dish and to wriggle their way 


36 H. H. LANE 


through an inch of cotton wool. At thirty hours of 
age the tendency to lie on the belly rather than on 
the side was more strongly marked than hereto- 
fore. They were rather restless, crawling about 
from place to place. Apparently voluntary scratch- 
ing movements with the hind foot were noted. The 
tail was held extended posteriorly, that is, was not 
tucked forward beneath the body as in the case of 
newly born rats. Frequent twitchings were noticed 
over various parts of the body, especially on the 
shoulders, hips, and flanks. In the case of 55-hour- 
old rats it was noted that when at rest they took 
various positions, but seemed to prefer to lie on the 
belly with the head held either in a straight line 
with the body-axis, or turned to one side, or even 
with the snout tucked down between the fore-legs. 
They were able to roll over voluntarily from one 
side to the other. Their movements were but little 
better co-drdinated than in the preceding stages. 
The 78-hour-old rats, when put into a Petri dish in 
the warm chamber lay flat on their bellies with the 
head extended in the line of the long axis of the 
body; the fore-limbs were spread more or less wide- 
ly apart, and usually the paw and the fore-arm to 
the elbow rested upon the supporting surface. The 
hind-limbs were also spread well apart but not so 
widely as the fore-limbs. Occasionally all four (4) 
limbs were drawn under the body in such a way as 
to hold it slightly elevated above the surface of the 
dish. The tail generally extended straight back- 
wards. They seemed rather “nervous,” frequently 
changing their position, twitching various parts of 
the sides and legs, moving the latter forward and 
backward, turning the whole body so as to face 
now in one direction and now in another, and the 
tail was occasionally directed forward so as to lie 
a'cngside the body. When rolled over on the back 


CORRELATION OF STRUCTURE AND FUNCTION 37 


or side, they usually remained comparatively quiet 
for a few seconds, and then rolled back and re- 
gained the usual position with the belly down. 

The young rats on the fifth day after birth had 
better co-6rdination of their movements than in pre- 
vious stages, though still far from complete. For in- 
stance, the left hind foot of one was much inflamed 
and swollen and the young rat spent much of his 
time at intervals in licking this foot. It was also 
observed deliberately washing its face by licking its 
perfectly normal front paw and then rubbing the 
face with it. These actions were performed not 
once but repeatedly. 

When placed with the hind quarters hanging 
down over the edge of a small box-lid, they made 
only feeble and futile efforts to keep from falling 
off. They would occasionally raise a hind foot as 
though to catch hold of the upper edge of the box- 
lid, but finally would usually fall off without any 
further attempt, apparently, to save themselves. 
Occasional ‘“‘stretching’’ movements were observed 
while the hind quarters overhung the edge of the 
box; they may have been attempts to regain a more 
comfortable position on top of the box-lid. There 
were also apparently voluntary attempts at scratch- 
ing the flanks with the hind feet. 

Nine-day-old rats crawled about with consid- 
erable agility; occasionally raised their heads to 
sniff. When at rest they lay on the belly with all 
four limbs spread well apart. When placed at the 
edge of the table top they moved along it with the 
feet and vibrissae of one side tracing the edge. 
When an attempt was made to push them head first 
over the table edge they braced themselves with 
their feet and pushed back with all their power. 
They righted themselves immediately when placed 
on their backs, although the movements were not 


38 H. H. LANE 


thoroughly co-drdinated, particularly those of the 
hind limbs and quarters. 

At twelve days of age the rats walked in the 
manner of an adult, though the movements were 
still lacking somewhat in co-6rdination; the latter 
was, however, noticeably better developed than 
heretofore. They hung back downwards from the 
experimenter’s finger, holding on with fore-paws 
and head, or with all four feet and head, for a few 
seconds, and then finally managed to pull them- 
selves over to the top side of the finger and thence 
to the back of the hand. When placed on a slide 
box they crawled around feeling the edge with their 
vibrissae and the ventral surface of the jaws. Some- 
times they would stick their heads far over the 
edge of the box and then would turn around and 
crawl to near the center, where they would remain 
until stimulated to further movement. Small noticed 
twelve-day-old rats crawling to the edge of a 
table stopping, reaching over as far as possible 
without falling, “throwing up the head and sniffing 
in the very characteristic way of rats when orienting 
themselves,” and then retreating. 

At sixteen days of age their movements were 
all well co-6rdinated. They crawled readily; their 
equilibrium was well established. They moved eas- 
ily from end to end along the experimenter’s finger 
without showing any sign of falling or of losing 
their balance. 


Structural Observations on the Organs of 
Equilibrium. 

In the 7144 mm. embryos there is no trace of 
semicircular canals. The auditory vesicle is large 
and spherical, its wall epithelial in character. There 
is a large endolymphatic duct running dorsad and 
ending blindly in the mesenchyme of the dorsal part 


=> te '. 


CORRELATION OF STRUCTURE AND FUNCTION 39 


of the head. The eighth nerve is a short trunk only 
that cannot yet be separated into its vestibular and 
cochlear portions, and none of its fibers could be 
detected reaching the cells composing the auditory 
vesicle. 

In the 16 mm. embryos the ear has reached a 
stage corresponding very nearly to that of the 20 
mm. human embryo, as described by Streeter. The 
three semicircular canals are well formed, and the 
vestibular nerve sends a completely fibrillated 
branch to the ampulla of the superior canal as well 
as one to the ampulla of the lateral canal, these 
two branches arising from a common trunk a short 
distance from their terminations. Shortly before the 
division just mentioned, the vestibular nerve gives 
off the utricular branch. A longer fourth branch 
runs to the ampulla of the posterior semicircular 
canal. The sacculus is a bud-like projection from 
the posterior side of the utriculus and from it the 
cochlea arises by a slightly constricted neck, the 
ductus reuniens. The cochlea extends in a generally 
ventrad direction, making one complete turn at its 
distal end. The innervation of the sacculus seems 
to be by a branch of the cochlear and not the vesti- 
bular nerve as Streeter maintains for Homo; at 
least the vestibular branch to the posterior semi- 
circular canal is not connected at this stage with 
the sacculus. The common trunk of the vestibular 
nerve emerges from the otic capsule and after 
reaching the brain cavity enters a ganglion (Acces- 
sory Ganglion?) from which it emerges before 
passing into the myelencephalon, to end in its nu- 
cleus close beneath the floor of the fourth ventricle. 
Distally the fibers of the vestibular nerve can be 
seen penetrating in among the cells that are be- 
ginning to elongate to become the sensory cells of 
the cristae acusticae. 


40 H. H. LANE 


In the 23 mm. embryo the cristae acusticae are 
prominent ridges about as high as broad at the 
base, and with the top of the ridge arched over in 
a very regular curve. The differentiation of the 
cells composing the epithelium covering this struc- 
ture has not gone far enough to enable one to dis- 
tinguish the sensory and supporting elements. How- 
ever, it is clear that there is an outer layer of cells 
rather regularly arranged everlying a basilar layer 
in which the cells have no very definite arrange- 
ment. A small amount of endolymph is present in 
the ampullae. The core of the cristae is made up 
of a mass of mesenchymatous cells among which a 
few unmedullated fibers of the vestibular nerve 
make their way to end among the cells of the epi- 
thelial layers. The latter, moreover, do not yet 
have the cilia or sensory processes found later. The 
vestibular nerve has the same relations with the 
medulla as those described for the 16 mm. stage. A 
tract from the same general region of the medulla 
in which the vestibular nerve ends runs dorsad into 
the cerebellum, but any actual relationship between 
the two could not be determined in these prepara- 
tions. 

In the rat at birth and during the first day the 
semicircular canals are much larger than in the 
preceding stages. The cristae acusticae are not only 
larger but their cells are differentiated into a su- 
perficial layer composed of stoutly columnar cells; 
and a supporting layer of very slender columnar 
cells, in many instances much longer than the sen- 
sory cells. Each sensory cell is inclosed in a “stock- 
ade’”’ of nerve fibers in such a way that a mechanical 
pressure exerted at any point must result in the 
stimulation of one or more nerve fibers. The pecu- 
liar terminal process projecting into the endolymph 
undoubtedly serves as a lever that magnifies the 


CORRELATION OF STRUCTURE AND FUNCTION 41 


sensitivity of the cell to movements of that fluid. 
Centrally the root of the vestibular nerve can be 
followed through the skull into the accessory nu- 
cleus thence on into the medulla, in the manner 
already described. Through the medulla its fibers 
course dorso-mesad, finally ending in a _ nucleus 
through which there also run correlational fibers 
caudad in the medulla and cephalad into the cere- 
bellum. Medullation has not occurred in any of these 
tracts. 

No further differentiations of any importance 
have been detected in connection with this appara- 
tus in later stages. 

The functions of the cerebellum, like so many 
‘other parts of the brain, are not thoroughly known; 
nevertheless it is generally agreed that the cere- 
bellum contains the center ‘‘for the maintenance of 
the mechani¢al equilibrium of the body” (Sherring- 
ton, p. 348). If this be true, it is evident that the 
central connections for the main organ of equili- 
brium, the system of semicircular canals, are estab- 
lished at or shortly before birth. At this time also 
the maculae and cristae acusticae have their char- 
acteristic structural features developed to a fune- 
tional extent. Were this the whole of the mechanism 
concerned in maintaining equilibrium it would ap- 
pear that the rat at birth could maintain its proper 
orientation without difficulty, and this is indeed true 
to a large extent. But the fact that the ability to 
maintain equilibrium improves during the succeed- 
ing two weeks or more of postnatal life indicates 
that other factors are involved. One of these is 
undoubtedly muscle tonus, which probably comes 
as an effect of use. Moreover, it will be recalled 
that it was only at the time when the eyes become 
functional that the power of equilibration is per- 
fected. This accords perfectly with the results of 


42 H. H. LANE 


investigations elsewhere on this sense. It is a well- 
established fact that the sense of sight has a very 
important relation to the maintenance of equili- 
brium. 


The vibrissae are also used as organs of orien- 
tation in the rat and constitute another element in 
the mechanism of equilibration. 


Summary of Results on Equilibrium. 


1. 744mm. embryos: 


a. There were no experimental results indicating a 
sense of equilibrum at this stage in development. 


b. There are no traces as yet of semicircular canals. 
2. 16mm. embryos: 


a. There was no experimental evidence of a sense of 
equilibrium in this stage. 


b. The semicircular canals are well formed, and the 
ampullae are innervated by fibrillated branches of 
the vestibular nerve. The region of the cristae 
acusticae is indicated merely by an elongation of 
the endothelial cells. 


3. 23 to 28 mm. embryos. 


a. There was again no experimental evidence of a 
sense of equilibrium in these stages. 


b. The differentiation of the cells of the cristae acus- 
ticae is proceeding, but the sensory and support- 
ing elements are not yet distinguishable. There 
are slight indications of a central connection with 
the cerebellum. 


4. 3.5 fetus: 


a. The earliest observed indication of a sense of 
equilibrium occured at this stage. One hour after 
removal from the uterus the young were able to 
maintain an upright position of head and body, 
and to regain this position when disturbed. When 


CORRELATION OF STRUCTURE AND FUNCTION 43 


turned over on the dorsum infrequent and feeble 
efforts were made to right themselves. 


The structural features are practically identical 
with those of young rats during the first day after 
birth. 


5. First day after birth: 


a. 


During the first day after birth the young rats 
crawled awkwardly about; turned the head from 
side to side; made awkward righting movements 
when turned over on the dorsum, which sometimes 
succeeded. 


The semicircular canals are larger than in the 
earlier stages described; the cristae acusticae 
have the sensory and supporting cells clearly dif- 
ferentiated; the former are inclosed each in a 
“stockade” of nerve fibers in such a way as to 
transmit easily any stimulus produced by a change 
in the position of the animal. The central con- 
nections of the vestibular nerve are well defined. 


6. Later stages: 


la. 


Throughout ‘the later stages there was mani- 
fested a gradual perfection in the sense of equili- 
brium, accompanied by a gradually increasing 
power of coordination of movements. 


The later stages witness the addition, through 
the establishment of various correlational tracts, 
of other factors concerned in the perfecting of 
the power of equilibration, notably (1) muscle 
tonus, (2) the use of the vibrissae, and (3) sight. 


44 H. H. LANE 


HXPERIMENTAL OBSERVATIONS ON THE 
SENSE OF SMELL. 


The youngest stage tested for the sense of 
smell was that of the 3.5 cm. fetuses. When re- 
moved from the uterus and the fetal envelopes they 
were placed in a dry dish in the warm chamber, 
and respiration was soon set up; the mouth opened 
widely and closed, as though yawning, or possibly 
gasping for breath—one could hardly decide which, 
possibly both. They wriggled and nosed one an- 
other about as though in an effort to find the moth- 
er’s nipples,— this occurred however only after the 
lapse of two hours or more from the time of their 
removal from the uterus. No perceptible reaction 
to various odors was detected, though when the 
brush dipped in an odoriferous substance acciden- 
tally touched the snout there was evident discom- 
fort manifested—probably a tactile response how- 
ever. 

Young rats during the first day after birth 
seemed to perceive odors as evidenced by turning 
the head and movements of the snout as though 
snifing. It was rather difficult to be sure of the 
results, however, since the responses occurred only 
after the lapse of a considerable time—15 to 30 
seconds—and may have been ‘“‘spontaneous” move- 
ments, i. e., due to other unknown stimuli. Small’s 
observations on this point are in the main corrobor- 
ative of the results recorded here. He notes: 
“Smell. 5 rats. All sensed violet, as indicated by 
expressive movements. Reaction, slow—about 15 
seconds. One only objected. All showed dislike to 
cheese, if movement away could be so interpreted. 
Instantaneous convulsive reaction to HCl.” 


CORRELATION OF STRUCTURE AND FUNCTION 45 


During the second day after birth no percepti- 
ble advance in the sense of smell was noted. The 
same turning movements of the head and twitching 
of the snout were evident, though the reaction time 
was still long—15 seconds or more—so that it was 
impossible to be sure of the relation of cause to 
effect. Small notes that in his rats there “‘seems 
to be slight advance in sense of smell, for they 
made no objection to the odor of cheese. Other 
odors elicited same responses as first day.”’ 

On the third day after birth the olfactory sense 
seemed a little better developed. The reaction time 
was shorter, ten seconds or less in some cases. Dis- 
tinct sniffing movements of the nostrils followed 
the presentation of a piece of cheese. Small re- 
cords for his animals at the corresponding stage 
that “only one of the five showed aversion to violet, 
and two to clove and asafoetida. Spirits of camphor 
and pennyroyal brought expressions of disapproval 
from all. Irritating fluid (HCl) produced instan- 
taneous responses from all. In addition to the motor 
reactions, there were vocal expressions and a strik- 
ing acceleration of respiration. Reactions to the 
other stimuli were slow, varying from ten to fifteen 
seconds.”’ 

On the fourth day after birth the olfactory re- 
sponses to various foreign odors (violet-water, cow’s 
milk warmed until it steamed, xylol, tobacco smoke) 
were more clearly defined than on the preceding 
days, except that the reaction time was if anything, 
longer, ranging from ten to twenty seconds, or even 
more. Small’s record for this stage is a follows: 

‘Smell. Reactions to violet, camphor, penny- 
royal, and clove, show less aversion; those to asa- 
foetida are quicker and show more dislike. In four 
cases out of five there seemed to be a pleasurable 
response to cheese-odor—in one case accompanied 


46 H. H. LANE 


by what sounded like a pleasant squeak. The fifth 
one paid no attention. In case of camphor and 
pennyroyal, it was easy to distinguish between the 
act of sensing the odor and the affective response. 
They sensed pennyroyal quickly—about 5 secs.— 
sniffed with deep respiration—then slowly averted 
the head.”’ 

Rats on the fifth day after birth when first 
removed from the nest were disappointing in their 
responses to odors. At times they seemed to dis- 
criminate between those which might be considered 
pleasant and unpleasant, and then again, showed 
utter indifference to them. On the whole the 
results were so contradictory that it was impossible 
to feel sure on this point. The reaction time in all 
cases was so long that one could not determine 
whether the movements may not have been due to 
other stimuli than those of odors. However, after 
they had been kept away from the nest several 
hours, and the mother had again been handled for 
a few minutes, they exhibited a noticeable increase 
in the degree of their activity upon being taken up 
in the hand for replacement into the nest. They 
“nosed”? around and nibbled at various places on 
the palm and fingers as though seeking the moth- 
er’s nipples. This may have been due to hunger 
and a feeling of warmth in the hand. SMALL’S 
record for the corresponding period in his observa- 
tions is terse—‘“‘nothing new in regard to the spe- 
cial senses.”’ However, on the 7th day he notes 
that ‘“‘the tests for smell seem to show a growing 
indifference to all but the positively painful stimuli 
—irritating fluids, e. g., HCl.” For the 8th day 
his statements appear rather contradictory; he 
says: “Reactions to odors become more individual. 
On the whole they tend to become indifferent. 


CORRELATION OF STRUCTURE AND FUNCTION 47 


Glacial acetic acid and carbolic acid gave negative 
reactions.” 

On the ninth day after birth the young rats, 
would raise their heads to sniff when tobacco smoke 
was blown over them. An extra strong puff from a 
pipe was followed by reflex movements and a weak 
regurgitation. They reached after a brush dipped in 
xylol and held one-fourth inch in front of the nose 
(they eyes were not yet open), until the head had 
been extended nearly half an inch directly for- 
ward; then they stopped and paid no further at- 
tention to the odor, withdrawing the head to the 
resting position. The snout at this time is well 
developed, having very much the form of that in 
the adult. SMALL records that during the 9th to 
llth days, “the special senses show no new fea- 
tures.”’ 

In twelve-day-old rats there was again a very 
marked olfactory response, when a brush dipped 
in xylol was held a short distance in front of the 
nose. The head was raised, turned from side to 
side, and the nostrils alternately dilated and con- 
tracted as though sniffing the odor. Upon gradu- 
ally removing the brush to a distance of six to 
eight inches, the rats moved forward a few inches 
(two to three) sniffing with the snout elevated as 
they moved. A similar response was made to the 
odor of alcohol. SMALL noticed twelve-day-old 
rats “throwing up the head and sniffing in the very 
characteristic way of rats when orienting them- 
selves.” On the fourteenth and fifteenth days, 
SMALL noted their ability to ‘sense odors at a much 
greater distance than previously.” 

By the sixteenth day young rats appear to 
have the sense of smell as well developed as have 
the adults. They move directly toward cheese and 
miscellaneous food stuffs put into the cage for the 


48 H. H. LANE 


mother to eat. SMALL notes that his rats at this. 
age “recoiled quickly from camphor. Moved quick- — 
ly toward brown-bread, dog-biscuit, and honey held 
at a distance of one inch. Appeared not to dislike 
iodoform or wintergreen.” 

In short, the sense of smell may be present, in 
a rudimentary form, at most, at birth or within a 
few hours thereafter. It is gradually perfected 
during the course of the first two and a half weeks 
of postnatal life as the rat’s relations to its environ- 
ment become more complex. Probably the earliest 
odor sensed under normal conditions is the body 
odor of the mother. Since the nest is saturated 
with that odor, and other surroundings have it to a 
less degree, the very young rats may be more 
strongly influenced to remain quietly in the nest 
during the mother’s absence than would otherwise 
be the case. At any rate such a hypothesis would » 
account for the very early appearance of this par- 
ticular sense, though of course temperature and 
contact sensations probably also enter into the sit- 
uation. 


Structural Observations on the Organ of Smell. 


In the 714mm. embryos the olfactory pits are 
well developed and open widely to the exterior. 
The future olfactory area is indicated by three 
pockets in the dorsal portion of the pit. There is 
no rhinencephalon nor an olfactory nerve. 

In the 16 mm. embryos the olfactory vesicle is 
large and the number of pockets in its dorsal area 
has increased to eight. The olfactory epithelium 
is much thickened in the olfactory area but other- 
wise no indication of the distinctive histogenesis of 
the olfactory cells is apparent. The olfactory 
nerve is present and has the form of a short brush 
of nerve-trunks converging to a small area of union 


CORRELATION OF STRUCTURE AND FUNCTION 49 


with the olfactory lobe. The latter is a short evag- 
ination from the anterior end of the prosencepha- 
lon and contains a large ventricular cavity, which 
opens widely at its posterior end into the lateral 
ventricle of the cerebral hemisphere. In the region 
of the olfactory vesicle the olfactory nerve 
branches are distributed to various parts of the 
olfactory epithelium and to Jacobson’s organ. From 
their earliest appearance the olfactory nerves differ 
greatly from all other nerves. The fibrillation is 
not so distinct and there is a large intermingling of 
mesenchymal (?) cells which in later stages con- 
stitute the sheath cells of the nerve branches; in 
fact, from the 28 mm. stage onward the fibrils are 
entirely inclosed by the sheath cells, giving the 
olfactory nerve a characteristic appearance by 
which it can be distinguished at a glance from all 
other nerves in the preparation. 

In the 23mm. embryo the fibrillation of the 
olfactory nerve is most clearly seen; in the 26 mm. 
embryo, the fibers are entirely inclosed by sheath 
cells except at the distal end of the most anterior 
branch. The sheath cells are elongated parallel 
to the long axis of the nerve and have distinctly 
elongated nuclei. The olfactory epithelium is still 
several cell layers thick but many of the cells 
whose distal ends constitute the surface of the 
olfactory pockets are becoming distinctly columnar, 
some at least extending fully half-way or more 
through the entire thickness of the epithelium. 
Their nuclei are elongated while those of the 
shorter more deeply situated cells are rounded or 
oval. In the 23 mm. embryo the cells of the rhinen- 
cephalon resemble mesenchymatous tissue, having 
large oval nuclei and numerous branching proto- 
plasmic processes. The definite formation of 
axones can be detected. In the 26mm. embryo, on 


50 H. H. LANE 


the other hand, the tractus lobi olfactorius is 
plainly indicated as a distinct bundle of non-med- 
ullated fibers, which runs from the anterior com- 
missure, of which it forms a part, on either side 
in a_ ventro-latero-anterior direction toward the 
olfactory lobe, which, however, it does not reach. 
Other olfactory tracts in the brain are not dis- 
tinguishable at this time. 

In the 3.5 cm. fetus the tractus lobi olfactorii 
is a rather large bundle of fibers that begins in 
the lobus olfactorius posterior, and runs in a dorso- 
posterior direction for some distance and then turns 
obliquely mesad, dividing into two smaller bundles, 
which reunite after running almost parallel for a 
short distance. The reunited bundle runs in a 
postero-dorso-mesal direction until it merges with 
the anterior commissure, of which it forms the 
anterobasal portion. It then passes across to the 
opposite side of the brain, where it leaves the com- 
missure and passes in an antero-latero-ventral direc- 
tion to the dorsal portion of the olfactory lobe of 
that side. Its distal end in both cases is enlarged 
and spread out into the form of a brush. Other 
olfactory tracts are not distinguishable. The olfac- 
tory nerve branches are distinctly associated with if 
not covered by the sheath ceils already described. 

In the rat during the first day after birth the 
olfactory epithelium comprises sustentacular cells 
many of which appear to be ciliated; a few typical 
olfactory cells are shown by the silver method. 
They are long and slender with relatively large 
nuclei and have a process from the basal end 
which enters the adjacent olfactory nerve branch. 
The tractus lobi olfactorii is somewhat larger and 
perhaps better defined than in earlier stages. It 
extends well forward into the olfactory lobe but 
not as yet into the bulbus. 


CORRELATION OF STRUCTURE AND FUNCTION 51 


By the third day after birth it has reached not 
only the bulbus but apparently to the region of the 
glomeruli. Its fibrils are more distinct, especially 
at its distal end than they were previously, and the 
whole tract is much larger. 

Succeeding days simply bring about the fur- 
ther development of these various parts along the 
lines already described. 


1 ay 


a. 


2. 3.d. 


Summary of Results on Smell. 


to 28 mm. embryos. 


No practicable means for testing the sense of 
smell in these earlier stages was devised. 


During these stages the olfactory apparatus is 
being gradually laid down, both as regards its 
central and its peripheral portions. The histo- 
logical differentiation of the olfactory epithelium 
has not advanced sufficiently far to enable the 
sensory cells proper to be identified. 


fetus: 


No absolutely certain response to olfactory stim- 
uli was obtained in this stage. 


Both the central and distal portions of the olfac- 
tory apparatus show appreciable development over 
the preceding stages, but sensory cells in the olfac- 
tory epithelium are not apparently fully differen- 
tiated. 


3. First day after birth: 


a. 


Apparent responses to olfactory stimuli were 
obtained in rats of this age, though the reaction 
time was long. 


The olfactory epithelium contains a few cells 
which are apparently fully differentiated as sen- 
sory cells. The central connections are better 
developed than before. 


52 H. H. LANE 


4. Later stages: 


a. There is, on the whole, a gradual perfecting of the 
olfactory sense from day to day. 

b. Pari passu, there is a gradual perfecting of the 
olfactory apparatus. 


CORRELATION OF STRUCTURE AND FUNCTION 53 


EXPERIMENTAL OBSERVATIONS ON THE 
SENSE OF TASTE. 


The 3.5cm. fetuses were able to make very 
feeble swallowing movements, but otherwise no 
results were obtained with liquids placed in their 
mouths. 

In the case of young rats during the first day 
after birth, various experiments were tried to test 
their sense of taste. With a saturated solution of 
cane sugar in tap water, presented on a camel’s 
hair brush, the first response was an attempt to 
push the brush away with the forepaws, probably 
on account of a tickling sensation when the brush 
touched the lips and snout. There was no audible 
squeaking noted. After the brush had been inserted 
into the mouth, they sucked away at it for several 
seconds, and upon the attempt being made to 
remove the brush, they held on with the jaws so 
firmly that the head and fore-quarters could be 
lifted from the dish in which they lay without them 
loosening their hold. 

With a saturated solution of sodium chloride 
in tapwater, there were evident signs of discom- 
fort displayed, and distinct attempts were made 
with the fore-paws to push the brush away. These 
movements were accompanied by quite audible 
squeaking. After the brush had been forcibly 
inserted into the mouth, sucking and swallowing 
movements followed, with no further evidence of 
discomfort. 

With a solution of 1% acetic acid in tapwater, 
the evidence of distaste or at least of annoyance 
was even more marked. No sucking movements 
could be perceived following the insertion of the 


54 H. H. LANE 


brush into the mouth; more persistent efforts were 
made to keep the mouth closed and thus to keep 
out the annoying object. The squeaking was 
louder and longer than in the other tests, and the 
movements of the fore-paws to push the brush 
away from the mouth were made with greater 
persistence and force. These experiments were all 
tried on several different individuals with like 
results in all cases. 

SMALL’S observations on rats of this age were 
as follows: 

“Taste. Tested with sugar-solution, warm 
milk, and strong salt solution. These were applied 
to the lips with fine brush. In each case, the rats 
squeaked and wiped at the offending stuff with fore- 
paws. Movements rather incoordinated. The move- 
ments are: brushing and pushing away with the 
fore-paws; averting the head; movement of the 
whole body. In case of the salt solution, the reac- 
tions were more vigorous, accompanied by voiding 
of urine. 

“Clear water called out the same characteris- 
tic reactions. 

“From this similarity of response, I infer that 
there is no differentiation of tastes, as pleasant and 
unpleasant. They are all unpleasant.” 

Rats of the second day showed no perceptible 
advance in taste over the previous day. 

On the third day taste seemed a little better 
developed; warm milk and sugar solution were 
received without protest and swallowing reactions 
followed. When the brush was wet in Ringer’s 
solution and applied to the lips the front legs were 
used in efforts to brush away the irritation, and no 
attempt to nurse could be detected. On the fourth 
day the gustatory responses could not be more 
clearly determined than on the preceding day. 


CORRELATION OF STRUCTURE AND FUNCTION 55 


On the fifth day, several liquid substances,—milk, 
sugar solution, salt solution, dilute vinegar, and 
even tap water—when presented to their lips on a 
brush caused expressions of discomfort, such as 
averting the head, wiping away the brush with the 
fore-paws, squeakings, and, if the stimulation was 
prolonged, wriggling away on the part of the 
whole animal. SMALL notes for the corresponding 
period: ‘‘Nothing new in regard to the special 
senses.”’ 

The nine-day old rats displayed nothing new 
in regard to the sense of taste; the same was true 
at twelve days of age. The seventeen day old 
rats pretended at least to eat various kinds of food 
that had been placed in the cage for the mother. 
Did not exhibit any particular choice as to what 
they tried; the whole performance may have been 
merely an imitation of their mother’s actions, for 
their eyes were now open and functional—a condi- 
tion not occurring in the previous stages. SMALL 
records for this stage that ‘‘one ate honey when 
a drop was put into his mouth. ‘Tried to gnaw 
brown-bread when a crumb was put into his mouth. 
After that when the brown-bread came _ within 
smelling range he would go toward it. Chewed 
a tiny piece, holding it in his paws in a well-bred 
rat’s way. I gave a little piece to another one. 
He took it in both paws and chewed it. The 
others scented it and tried to help, but he quickly 
drew away with his treasure. There seems to be 
immediate association between smell and taste. 
Though not conclusive, the evidence points that 
way. Another one declined to eat  sealing-wax 
after smelling it, and spat it out when a piece was 
put into his mouth.” 


56 H. H. LANE 


Observations on the Organs of Taste. 


In the 7144 mm. embryos no trace of any part 
of the glossopharyngeus nerve could be detected 
running to the mandibular arch. The mandibular 
branch of the trigeminus is a large bundle of fibres, 
which ends as a well-defined brush in the mesen- 
chyme of the mandibular arch. A branch of the 
facial nerve runs into the base of the hyoid arch, 
and is likewise fibrillated. 

In the 16mm. embryos the lingual branch of 
the glossopharyngeus nerve runs to the posterior 
part of the tongue and its fibers are distributed 
among the muscles of the superficial layer. The 
mandibular branch of the trigeminus innervates, so 
far as can be determined, the rest of the tongue. 
No indications of taste-buds are present. Over the 
surface of the anterior two-thirds of the tongue, 
however, there are at least ten longitudinal rows. of 
dome-shaped papillae, each consisting of a single lay- 
er of cubical cells forming the dome, and in the nearly 
spherical central cavity of the papilla there is a 
small number of nearly spherical cells. In a few 
instances nerve fibers can be detected entering the 
open base of the papilla and ending in a glomer- 
ulus or plexus around the central cells. A _ single 
median circumvallate papilla is fairly well-defined 
on the posterior portion of the tongue, and in this 
there is a relatively large plexus of nerve-fibers 
belonging to the glossopharyngeus. No taste-buds 
can be detected in the epithelial covering of this 
papilla. | 

In the 23mm. embryo conditions are practi- 
cally the same as those just described, allowing, of 
course, for an increase in size in all the parts men- 
tioned. In the circumvallate papilla, some of the 
nerve fibers now extend toward, if indeed they do 


CORRELATION OF STRUCTURE AND FUNCTION 57 


not end among the epithelial cells of its surface. 
No taste-buds could be demonstrated. Over the 
anterior two-thirds of the upper surface of the 
tongue the dome-shaped papillae are present and 
show little if any advancement over the preceding 
stage described, except an increase in size. 

In the one-day-old rat these dome-shaped 
papillae are much larger in size than before; their 
outermost layer of cells is somewhat flattened and 
covered with a thin cuticle; the inner cells are tak- 
ing on a form and arrangement suggestive of a 
taste-bud, but only one such structure is present in 
each papilla, and that is situated in the center of 
the distal surface of the papilla. Nerve fibers run 
in among the central cells. 

In the five-day old rat the dome-shaped papil- 
lae are larger and the number of nerve fibers run- 
ning to each is much greater than before. Other- 
wise they appear very much the same. 

In the nine-day old rat, however, quite a 
marked advance can be seen. The papillae on the 
whole are larger; their surface epithelium is much 
thinner and arched into a dome and in its center 
has appeared a small orifice surrounded by special 
cells. Beneath this outer layer, the cells of the 
stratum germinativum are elongated and arranged 
in somewhat the same manner as the elements of a 
taste-bud. The innervation is by fibrils of the trig- 
eminus. At this time also the circumvallate pap- 
illa has numerous taste-buds of the usual type 
lying within its epithelial layer. Taste-buds 
also occur in the walls of the outer margin of the 
groove surrounding the circumvallate papilla. 


58 


H. H. LANE 


Summary of Results on Taste. 


Before Birth: 


NS 


The 3.5 cm. fetus were able to swallow, but neither 
in them nor in any preceding stages were there 
obtained any evidence of a sense of taste. 


At no time previous to birth could taste-buds or 
other fully differentiated organs of taste be dem- 
onstrated. 


First day after birth: 


a. 


The results of tests for a sense of taste at this 
stage were very uncertain; apparently anything 
applied to the mouth produces a sense of discom- 
fort. Sugar-solution, however was received with 
much less objection than salt or acid solutions, 
and may possibly have been perceived as having 
an agreeable taste. 


True taste-buds are not demonstrable in the pre- 
parations in hand; the dome-shaped papillae 
(fungiform) over the anterior part of the tongue 
are developing an organ of sense faintly sugges- 
tive in its general form and arrangement of a 
taste-bud, though decidedly not a typical one. 


Older stages: 


a. 


Though it was exceedingly difficult to distinguish 
between annoyance or discomfort and a sense of 
taste, it was apparent, especially in the later 
stages, that this sense was present and gradually 
being perfected. 


There is likewise a gradual increase in the histo- 
logical differentiation of the organs of taste until 
by the ninth day, at least, taste-buds are distinctly 
formed on the sides of the circumvallate papilla, 
and a decidedly different organ in the dome-shaped 
(fungiform) papillae. 


CORRELATION OF STRUCTURE AND FUNCTION 59 


HXPERIMENTAL DATA ON THE SENSE OF 
HEARING. 


Absolutely no response to sound was noted 
before the twelfth day after birth. At that time 
a sharp clapping of the hands occasionally seemed 
to produce a response, i. e., the raising of the 
pinnae and turning of the head so as to face the 
direction of the sound. At other trials there was 
no apparent response. The same results were 
obtained by the ringing of a small hand bell. The 
shrill sound made by drawing in the breath sharply 
between the nearly closed lips several times was 
followed by a “‘nervous start,’ quite as characteris- 
tic but not quite so pronounced as that made by 
much older rats. 

On the sixteenth day hearing is well estab- 
lished. Previous to this time the external auditory 
meatus is more or less closed by a cellular plug 
which would effectually obstruct the passage of all 
sound waves except in the case of very loud or 
very shrill noises. Attempts to remove this obstruc- 
tion always resulted in so much hemorrhage and 
pain, or in so much damage to the ear, that no suc- 
cess was attained in attempts to secure an unob- 
structed passage for sound waves previous to the 
time when the meatus opens by natural means, i. e., 
the degeneration of the cells composing the plug. 

SMALL’S observations and those of WATSON also 
are in complete agreement with those recorded here. 

SMALL’s record is as follows: 

“Hearing. The bursting of a bag three feet 
away caused them to jump quite out of the nest. 
Later, clapping hands sharply at a distance of 10 
feet caused the quick recoil peculiar to rats. Did 


60 H. H. LANE 


not run. A sharp “sh” at 3 ft. brought their heads 
up. Word “rats” in a low tone at 1 ft. caused a 
slight jump. Rustling of paper produced the same 
result. Whistling brought up the head as if listen- 
ing. Even at the very dawn of ear-consciousness 
there seem to be differences of emotional reaction 
to different elements in the ‘big buzzing confusion’ 
around them. Every concussion elicits a startled 
movement; the gentle, prolonged note, e g., 
whistle, on the contrary, produces a reaction indi- 
cative of unscared attention.”’ 


Observations on the Organ of Hearing. 


In the 744mm. embryos the auditory vesicle 
is large and spherical with an epithelial wall and a 
well-defined endolymphatic duct running dorsad 
and ending blindly in the mesenchyme of the dor- 
sal part of the head. The acustico-facialis gang- 
lion is a large and definitely delimited mass of 
cells, with numerous fibres connecting it with the 
myelencephalon but no fibres have as yet reached the 
cells composing the wall of the auditory vesicle. 

In the 16 mm. embryo the ear has developed to 
the stage very nearly corresponding to Streeter’s 
20mm. human embryo. The endolymphatic duct is 
long, slender, and ends distally in an enlarged sac- 
cus endolymphaticus. The utriculus and sacculus 
are distinctly formed, the latter being a bud-like 
projection from the posterior side of the former. 
From the sacculus, the cochlea arises by a slightly 
constricted neck, the ductus reuniens, and extends 
generally ventrad, making one complete coil at its 
distal end. The spiral ganglion extends along the 
median one-half or two-thirds of the cochlea, or at 
least it cannot be detected for some considerable dis- 
tance from either the proximal or distal ends of the 


CORRELATION OF STRUCTURE AND FUNCTION 61 


cochlea. It is made up of ovate bipolar cells. Those 
in the distal portion of the ganglion have as yet no 
fibrillated processes; those more proximal in posi- 
tion have fibrillated processes at both poles, but 
the distal ends do not enter the epithelium of the 
cochlear canal, though in places they come into 
contact with the cochlear epithelium. No trace of 
an organ of Corti can be seen. 

In the 23 mm. embryo, the organ of Corti is 
indicated merely by two low ridges in the endo- 
thelial lining of the cochlear canal. One of these 
is broader than the other and represents the begin- 
ing of the membrana tectoria and adjacent parts; 
the other more narrow will become the sensory 
portion of the organ of Corti. Except that the 
cells of this region are columnar and longer than 
those elsewhere lining the canal, no histological 
differentiation can be perceived. These ridges are 
present only in the proximal portion of the coch- 
lear coil, not yet having appeared toward the dis- 
tal end, except as indicated by a general thickening 
of the endothelium on one side of the cochlear 
canal. 

In the one-day old rat the external auditory 
meatus is indicated in the sections as a long flat- 
tened and folded tube whose lumen is entirely 
obliterated by a plug of cells similar in all respects 
to those which compose the raphe palpebrarum 
or area of separation between the eyelids before 
the latter are open. The organ of Corti shows 
an advance in size over the preceding stage 
described; the membrana tectoria is rather well 
developed. The ridge that will give rise to the 
sensory portions of the organ of Corti is well 
marked but differentiation into hair-cells and rods 
has not yet occurred. The organ is not equally 
well developed throughout its whole extent, the 


62 H. H. LANE 


median portion showing greater differentation than 
either end. The limbus spiralis has made its 
appearance. The fibers of the cochlear nerve have 
not yet established any visible relation with the 
sensory portion of the organ of Corti. 

In the five-day-old rat the external auditory 
meatus is not only larger but its walls are still more 
complicated by the development of folds or ridges, 
so that a cross-section of it may be \Y-shaped, Y- 
shaped, oo-shaped, or i shaped. Along its cen- 
tral extent a lumen is beginning to appear through 
the disintegration of the cells composing the raphe; 
at either end it is still plugged with a solid mass 
of ectodermal cells. Internally the scala vestibuli, 
scala tympani, and cochlear duct are all present; 
the organ of Corti is larger and differentiation of 
the hair-cells and rod-cells is beginning though 
they are not distinct as yet. 

In the nine-day old rat the limbus spiralis is 
well defined; the scala vestibuli and scala tympani 
are much larger than in the preceding stages. 
There is no vas prominens; the ligamentum spirale 
is well developed; the lamina spiralis membrana- 
cea is complete; the lamina spiralis ossea is not yet 
even chondrified. Hensen’s cells are very large, 
shortly columnar, and 4 or 5 in number in each 
section; a row of cubical cells (the cells of Clau- 
dius) are distinguishable from the adjacent endothe- 
lium. The sulcus spiralis has not yet formed. The 
cells of the organ of Corti proper have not attained 
their definitive differentiation; however, neurofi- 
brils from the ganglion spirale reach their bases. 
The lumen of the external auditory meatus is still 
obliterated both proximally and distally by the 
plugs of ectodermal cells. 

In the thirteen-day old rat the ear shows a 
general advance in all its parts. The vas promin- 
ens has developed; the lamina spiralis ossea is 


CORRELATION OF STRUCTURE AND FUNCTION 63 


chondrified. The organ of Corti has differentiated 
fully for at least the greater part of its extent. 
The tunnel of Corti is large and bounded by the 
inner and outer supporting rods. The inner and 
outer hair cells can be seen with the fibrils from the 
ganglion spirale ending about them. The lumen 
of the external auditory meatus is more or less 
open throughout its whole length, though the detri- 
tus of the old cellular plug still remains. The 
latter is however no longer cellular, but rather 
seems to have undergone liquefaction, and at no 
point apparently completely fills the meatus. The 
structural conditions at this stage would indicate 
the possibility of the perception of some sounds at 
least. 

In the sixteen-day old rat, aside from growth 
in size, the conditions are _ practically those 
described for the thirteen-day old rat. The detri- 
tus in the meatus has mostly disappeared. The 
organ of Corti has differentiated for most or all 
of its extent. So far as the structural conditions 
are concerned, the apparatus would appear to be 
able to respond to any sound stimulus. 


Summary of Results on Hearing. 


1. Absolutely no response to sound was noted before 
the twelfth day after birth. From that date until 
the sixteenth or seventeenth day there is a gradual 
increase in the ability to perceive sound. 


2. Previous to the twelfth day the portions of the ear 
concerned with the perception of sound have been 
undergoing a gradual development but had not yet 
reached that degree of differentiation of the organ 
of Corti necessary for the perception of sound. By 
the twelfth or thirteenth day, the organ of Corti 
is apparently differentiated for at least part of its 
extent, though the lumen of the external auditory 
meatus is not fully opened. The next few days 
witness the completion of the differentiation of the 
apparatus of hearing. 


64 H. H. LANE 


HXPERIMENTAL DATA ON THE SENSE OF 
SIGHT. 


Absolutely no response to light was obtained 
before the opening of the eyes on the sixteenth or 
seventeenth day. At practically all the preceding 
stages the eye had been tested with an electric 
flash-light without appreciable result. 

On the sixteenth day one only out of a litter 
of six had its eyes open and functional; it re- 
sponded quickly to movements made before its 
face and turned its head from side to side to follow 
moving objects with its eyes. The others did not 
get their eyes open until the seventeenth day, at 
which time sight was fully established in them also. 

SMALL says of his experiments on this stage: 

‘Sight. When brought into a strong light they 
did not wink or show uneasiness, though they soon 
closed their eyes, and seemed to become drowsy. 
A stroke of the hand one inch in front of the 
face caused winking and a slight recoil of the 
head.” 


Observations on the Organ of Sight. 


In the 714 mm. embryo the eye is in the 
stage of the optic cup; the lens is a hollow vesicle 
lying deeply within the mouth of the cup. There 
is no apparent differentiation in the retinal layer. 
The area between the lens and the ectoderm is filled 
with loose, spongy mesenchyme. The choroid fissure 
is not closed. No fibrillation can be detected in the 
optic stalk, i. e., there is no optic nerve as yet devel- 
oped. 

In the 16mm. embryo the lens is a solid oval 
body with the diameter in the future pupillary 


CORRELATION OF STRUCTURE AND FUNCTION 65 


axis somewhat the longest. Its anterior surface 
consists of a relatively thin layer of columnar cells. 
The margin of the optic cup shows a decided but 
rather gradual thinning out of the retinal layers, 
but no other indication of the ciliary body is appar- 
ent. The surface of the retina toward the cavity 
of the cup is supplied by a rich plexus of small 
blood-vessels and the fibrillar layer of the retina 
has begun to differentiate. The remainder of the 
retina shows no further differentiation into distinct 
layers. Its nuclei are relatively large and oval 
in outline; they are apparently more numerous 
than in the preceding stage. 

The surface of the lens is covered with a 
very rich plexus of small blood-vessels. The cham- 
ber of the vitreous humor is relatively small and 
shallow, and contains a small amount of substance 
that in the preparations has the appearance of a 
sparse network of fibers, somewhat like a _ very 
loose mesenchyme minus the nuclei. There is no 
anterior chamber. The area between the lens and 
the ectoderm is filled with a rather densely packed 
mesenchyme, in which on the side next to the lens 
numerous blood-vessels can be seen. The develop- 
ment of the eyelids and socket is indicated by the 
presence of a groove on the surface of the head 
surrounding the optic area and dipping in to a 
distance of one-third or more around the optic 
cup. The arteria centralis retinae is a relatively 
large and distinct vessel forming a central core in 
the optic stalk where the latter joins the optic cup. 
Less than 180 » outside the cup the artery emerges 
from the stalk through the remnant of the choroid 
fissure and thence posteriorly the two are inde- 
pendent of each other. The cavity of the optic 
stalk is still present and while throughout about 
one-half of its length its walls are in contact with 


66 H. H. LANE 


each other, still it can be easily traced into the 
ventricle of the brain. The optic nerve is develop- 
ing in the form of numerous fibers in the ventral 
portion of the optic stalk. Cross sections show it 
to be large toward its retinal end, indicating that 
processes from the retinal cells are growing toward 
the brain. 

In the 23 mm. embryo, the lens is more nearly 
spherical, if anything the diameter in the pupillary 
axis is the shorter. The anterior surface consists 
of a layer of columnar cells. The posterior surface of 
the lens is covered by a rich plexus of small blood- 
vessels. The retinal wall is thicker than in the 
preceding stages. The fibrillar layer is better 
developed. Many of the cells of the retina proper 
are columnar and some extend nearly or entirely 
through that structure. The third of the retina 
nearest to the vitreous humor is made up of cells 
oval in form and not densely packed together; the 
other two-thirds is composed of cells more colum- 
nar in form and more closely packed together. The 
cavity of the vitreous humor is a little larger than 
in preceding stages and has, in the preparations, 
the same sort of a reticular content through which 
passes a cone-shaped plexus of small blood-vessels 
from the optic nerve to the lens. The ciliary body 
is indicated by a very decided thinning out of the 
wall of the optic cup, and consists of columnar 
cells arranged in one plane, i. e., none of the folds 
so characteristic of the ciliary body have yet begun 
to form. The region of the future cornea and 
anterior chamber is occupied by a rather densely 
packed body of fibrous mesenchyme. Between this 
and the eyelids, which now cover the eye com- 
pletely, there is a distinct cavity. The eyelids have 
fused into a continuous layer but the line of their 
future separation is indicated by a raphe bounded 


CORRELATION OF STRUCTURE AND FUNCTION 67 


on either side by a row of columnar ectodermal 
cells, the stratum germinativum of the margins of 
the lids. The optic stalk is wholly replaced by 
the optic nerve, that is fibers from the retina extend 
entirely to the brain. The arteria centralis retinae 
is relatively smaller than in preceding stages and 
enters the optic nerve almost at the exact point 
where the latter enters the eyeball. 

In the rat during the first day after birth the 
shape of the lens is about the same as in the pre- 
ceding stage described. The epithelial layer over 
its anterior surface now extends about two-thirds 
of the way around it and consists, at least in its 
most anterior area, of cells which are cubical or 
even slightly flattened instead of columnar. The 
surface of the lens is still richly supplied with a 
plexus of small blood vessels. The vitreous humor 
is greater in amount and in the preparations 
appears more granular or homogeneous rather 
than reticular, and through it there runs the coni- 
cal plexus of bloodvessels from the retina to the 
lens, already mentioned. The ciliary body has two 
distinct folds with slight traces of others. The 
retina exhibits indications of six or seven different 
layers though its histological differentiation is oth- 
erwise hardly more than begun. There is no ante- 
rior chamber; the region of the cornea is thick 
and composed of a densely packed fibrous mesen- 
chyme. The eyelids are much better developed 
than heretofore though still united by a raphe of 
ectodermal cells. Anlagen of eyelashes are present 
in considerable numbers. Glands are making their 
appearance along the inner margins of the lids in 
the form of tubular ingrowths of epidermis, while 
in the mesenchyme, sphincter and other muscles 
are in process of differentiation. Within the brain 
the optic tracts are a distinct bundle of fibers that 


68 H. H. LANE 


pass up through the optic thalami into the anterior 
corpora quadrigemina. Many other tracts are pres- 
ent also, but in the absence of medullation it is 
extremely difficult to trace them with certainty; 
however, it seems probable, to say the least, that 
by birth or shortly thereafter all the tracts within 
the brain connected with the primary optic centers, 
i. e., those in the corpus geniculatum laterale, the 
superficial portion of the anterior corpora quadri- 
gemina, and the pulvinar, are laid down. The 
relations of these centers to cortex of the occipital 
lobes could not be made out. 

In the five-day-old rat the lens has practically 
the same structure as at birth. The plexus of 
bloodvessels on its posterior surface is still well 
marked. The cavity of the vitreous humor is some- 
what larger and its contents somewhat greater and 
denser than in earlier stages. Differentiation of 
the retina is proceeding but rods and cones are not 
yet distinguishable. The ciliary body exhibits an 
increased number of folds. The anterior chamber 
is present and the iris is beginning to be formed. 
The development of the lids shows a decided 
advance in every respect, but the raphe is still a 
thick layer of cells. 

In the nine-day old rat the optic nerve and 
the lens are practically in their definitive condi- 
tion, though the latter still is supplied with its 
plexus of blood vessels over its entire posterior sur- 
face. The ciliary body has at least seven folds; 
the anterior chamber and the iris are better devel- 
oped than before. The cavity of the vitreous 
humor is much more extensive. All layers of the 
retina are distinguishable, the rods and cones ap- 
parently being in process of formation. 

In the twelve-day-old rat the most noticeable 
advances are to be found in the ciliary body and 


CORRELATION OF STRUCTURE AND FUNCTION 69 


the retina. The former is distinctly marked off 
from the latter, in which the rods and cones are 
fairly well defined. The lens still possesses its 
plexus of blood vessels as heretofore described. The 
cornea and the secondary structures in that region 
are all in process of advanced development. The 
lids are however still closed and no light can reach 
the eye. 

In the sixteen-day old rat (frequently not until 
the seventeenth day), the cells of the raphe palpe- 
brarum have degenerated and the lids are separ- 
ate. The rods and cones are much better differenti- 
ated than heretofore, and are undoubtedly functional ; 
ali the other retinal elements are also apparently fully 
formed. The plexus of bloodvessels to the lens 
is still present, though much reduced. 


Summary of Results on Sight. 


1. Absolutely no response to light was obtained before 
the opening of the eyes on the sixteenth or seven- 
teenth day. 


2. Before the twelfth day after birth the eye is under- 
going the usual course of development. At this time 
(twelfth day) the rods and cones are fairly well 
defined, but the accessory structures are less fully 
developed and the closed lids prevent the entrance of 
any but possibly the very brightest light. By the 
sixteenth or seventeenth day, the lids open and the 
function of sight is fully established. 


70 H. H. LANE 


THE CAUSES OF DEVELOPMENT AND OF 
DIFFERENTIATION IN THE NERVOUS 
SYSTEM. 


One of the most striking facts in connection 
with this investigation is one which must have been 
forced upon the attention of all who have studied 
the interrelations of organs in the development of 
the embryo of any vertebrate species, namely, the 
early appearance of the peripheral portion of the 
nervous system. At a time when it is inconceiv- 
able that distinctly nervous functions can be possi- 
ble or at least of any importance to the embryo, 
the chief nerve trunks are all laid down, together 
with most or all of their important branches. For 
example, the vestibular and cochlear nerves are 
well developed in the 23mm. rat fetus, not to 
speak of still earlier stages, while it is absolutely 
impossible that the function of hearing can have 
been established. Indeed, if our experiments can 
be relied upon, the very first indication of an abil- 
ity to detect sound comes not earlier than the 
twelfth day after birth. Yet here in the fetus which 
has passed through only about two-thirds of its pre- 
natal life, the nerve of hearing is apparently fully 
formed, at least fibrillation is complete, and this 
as we have seen is most certainly to be regarded 
as an indication of the establishment of the power 
of functional activity. 

The anlagen of the vibrissae in the 16mm. 
embryo have not more than reached the surface 
of the epidermis of the snout and it can hardly be 
supposed that the fetus has need of a delicate sense 
of touch to maintain itself within the amniotic sac. 


CORRELATION OF STRUCTURE AND FUNCTION 71 


And yet, the maxillaris and mandibularis branches 
of the trigeminus are completely fibrillated and end 
in very large and complex basket-like networks 
in the follicles of the vibrissae. It is inconceivable 
that in these and other cases that might be cited 
the nerves and end-organs develop in response to 
functional activities or even functional needs on the 
part of the fetus at this or any preceding stage in 
its existence. The condition as stated exists, how- 
ever, and demands an explanation. 

The earliest stages in the development of the 
peripheral nerves have been studied experimentally 
by HARRISON (710) and others, and the results 
obtained, especially from the cultivation of tissues 
im vitro, shed a flood of light upon the question 
raised above. HARRISON finds that all _ tissues 
exhibit a specificity in their tendency to undergo 
each its own peculiar type of histogenesis, as the 
result of which certain cells in vitro become muscle 
cells, others epithelial, others connective tissues, 
others nervous, etc. This tendency is inherent in 
the cells concerned and reveals itself irrespective 
of the nature of the external conditions, so long as 
the latter are not detrimental to the well-being of 
the cells themselves. In short, a neuroblast is 
potentially a nerve-cell long before it is definitively 
such, owing to an internal organization that has 
been handed down to it through all the cell-genera- 
tions that have intervened between the neuroblast 
stage and the odsperm. Indeed, it is not altogether 
a mere inference, as the work of Whitman, Wilson, 
Conklin, Lillie, and others, has shown that its pos- 
sibilities were predetermined or prelocalized in the 
egg at or before fertilization. In short the devel- 
opment of the nervous system in general, and the 
differentiation of its constituent parts in particular, 
as is likewise true of all other organ systems as 


72 H. H. LANE 


well, are the products of a predetermination in the 
odsperm; a process of endogenesis, as Conklin has 
termed it, and not of epigenesis. 

Furthermore the fate of each neuroblast and 
its products is likewise predetermined and there 
follows the histogenetic differentiation of the neu- 
rones of the central nuclei, motor or sensory as the 
case may be, and of the peripheral ganglia, as the 
result of an “immanent force” that needs no direct 
outside stimulus for its production. Thus HAR- 
RISON shows in the case of the formation of the 
axone that the outgrowth takes place: 


“without the application of any external phy- 
sical force and . . . . occurs even when 
the normal surroundings are radically modi- 
fied. That the original direction taken by the 
outgrowing fiber is already determined for 
each cell before the outgrowth actually begins, 
so that when it does begin it is dependent 
upon forces acting from within, follows first 
from the fact that the nerve fibers within the 
embryo tend to grow out in a given direction 
even when quite different surroundings are 
substituted for the normal, and secondly, from 
the fact that the nerve fibers which grow into 
the clotted lymph, are there surrounded on all 
sides by an isotropic medium, which cannot 
conceivably be held to produce movement in a 
definite direction.”’ 


In other words these structures are repre- 
sented by something in the odsperm, whose nature 
can only be conjectured, and they appear not as 
direct responses to the needs of the embryo, but in 
anticipation of those needs, because of the inherited 
tendencies and forces immanent in the odsperm and 
localized as development proceeds in the parts con- 


CORRELATION OF STRUCTURE AND FUNCTION 73 


cerned. They are “racial or inherent adaptations 
which are not first called forth by the contingent 
stimulus to which they are the appropriate and 
useful response” (Conklin (’15) ). 

This early establishment of peripheral con- 
nections on the part of the nervous system receives 
its proximate explanation in certain mechanical 
conditions that exist at an early stage in embryonic 
development but not later. Assuming the truth of 
the neurone hypothesis, the question of how any 
certain nerve reaches unerringly its proper termina- 
tion,—a question that has provoked much discus- 
sion,—receives an easy answer. 

HARRISON’S experiments show that each neu- 
rone sends out its axone in a predetermined manner 
and direction; that this axone is in the form of a 
protoplasmic process or pseudopodium which ex- 
tends outward from the neuroblast toward its peri- 
pheral termination; that this process grows from a 
terminal bulb—Cajal’s coéne d’accroisement—which 
with its changing pseudopods reaches out constant- 
ly in various directions, but ultimately extending 
through a distance of a millimetre or more until it 
reaches the muscle-plate or epithelium with which 
it is destined to connect. That this activity must 
take place early in embryonic life 1s naturally what 
one would expect, since it is only in these early stages 
that the neuroblasts of the neural tube lie within the 
specified distance—about a millimetre or less—from 
the parts they are destined to innervate. On the 
basis of adaptation and natural selection it is plain 
that only those embryos that thus early establish these 
connections can develop properly and so survive. 

As HARRISON points out very clearly from his 
own results, and as has been shown by the observa- 
tions recorded above on the white rat, the neuro- 
blasts that thus early come into direct relation with 


74 H. H. LANE 


their peripheral end-organs are relatively few in 
number, but having once made the connection they 
elongate whenever and wherever needed as the 
growth and shifting of parts goes on so that when 
the ultimate relationships have become established 
the nerve paths have also been marked out, and 
later nerve processes growing out from neighboring 
neuroblasts, in relation to the greater’ functional 
needs of the embryo or as opportunity is afforded 
them, find their course already determined for them 
and have no trouble in reaching their own particu- 
lar end-organs. This early growth period of the 
neuronal processes is clearly a stereotropic response, 
as HARRISON’S work shows; the later connections 
of the fibers with the individual cells of the end- 
organs is probably due, as HARRISON says, to 
chemotaxis. It is hardly possible on any other 
grounds to explain how it comes about that where 
both sensory and motor fibers pass out in the same 
nerve trunk the latter turn aside to terminate in 
muscle cells, while the former pass on their way to 
end in epithelial sense cells. 


CORRELATION OF STRUCTURE AND FUNCTION 175 


CORTICAL CONNECTIONS IN THE RAT. 


The cortical connections with the lower centers 
of the brain in mammals are chiefly made through 
the corona radiata. In this, fibers from all parts of 
the cortex are gathered together and pass caudad 
into the thalamus, the cerebellum, the medulla, and 
the cord; without doubt other fibers arising in the 
lower centers pass cephalad via the same route to 
reach the cortex. In the higher mammals there are 
other bundles confined to the hemispheres which 
connect more or less distant parts of the pallium 
with each other. In the rat, however, in common 
with many others of the lower mammals, these cor- 
tical association tracts are very poorly developed. 
The corona radiata on the other hand is a promi- 
nent structure even at birth and it may be assumed 
from the known relations in higher mammals and 
man that its presence may be regarded as conclu- 
Sive evidence that connections between the lower 
centers and the cortex have already been estab- 
lished. While there are probably no medullated fiber 
paths present in the brain of the rat at birth, nor 
indeed for several days after that event, neverthe- 
less the medulla, the cerebellum, the mid-brain, and 
the diencephalon contain many non-medullated fib- 
ers, and the optic tracts, the olfactory tracts and 
lobes, the anterior, posterior, and habenular com- 
missures, the corpus callosum, the external capsule 
and the deeper layers at least of the cortex exhibit 
the same condition. Hence, in view of the sparse- 
ness of association tracts in the cortex of the adult 
rat, and of the facts just stated, it seems not im- 
probable that most of the associations possible later 
in life are already established at birth or very soon 


76 H. H. LANE 


thereafter. Certainly by the tenth day after birth 
the cortical connections are pretty definitely estab- 
lished, though owing to their primitive and probably 
rather diffuse character it has not been possible 
with my preparations to map them out. This is 
probably due to the lack of medullation of these 
fibers, and might be considered as vitiating our con- 
tention. However, WATSON (’03) has shown con- 
clusively that such an objection has no force in this 
connection. Briefly put WATSON found that though 


“at birth (and during the first twenty-four 
hours after birth) the rat is not only capable 
of making many co-6rdinated movements, but 
is also capable of receiving sense impressions 

. no medullated fibers are present in 
either the peripheral or the central nervous 
systems.”’ 

Furthermore, during the first day after birth, 
“there must be, too, some pathway between 
sensory and motor nerves, because the rat 
moves when his tail is pinched, sucks when the 
stimulus of the mother’s teats touches his mouth, 
scratches his nose with his forepaw when he 
smells something unpleasant. . . Granting 
now (and the evidence seems conclusive) that 
we have motor responses to sensory stimuli at 
birth, we must admit a pathway from skin to 
muscle. Such a pathway involves peripheral 
sensory neurones, central neurones, and finally 
motor neurones. During the first few days, at 
least, impulses must travel over the unmedul- 
lated axis cylinders of all these neurones. Co- 
Ordination in the movements mentioned above 
grows rapidly better. At eight days 
the rats are able to crawl vigorously and, 
when crawling to show some selection of path 


CORRELATION OF STRUCTURE AND FUNCTION 77 


by sniffing and going in different directions. 
: Sensitivity for smell, taste, and dermal 
stimuli has increased rapidly since the first day. 

Whether or not at eight days the 
cortex is necessary for the responses which the 
rats make to the various stimuli may be a 
question. (If the rats were really smelling out 
a path, it would of course be necessary.) But, 
assuming that the cortex is not involved in 
these movements, we still have to account for 
the neural pathway in the lower centers over 
which these impulses can travel. Granting that 
the fibers carrying the impulses from a given 
sensory area are all medullated, and granting 
that the motor fibers which go to the corre- 
sponding muscles in any particular case .are 
also medullated, if nevertheless medullation is 
lacking in some or all of the pathways within 
the central nervous system, then, so far as the 
physiological reaction taken as a whole is con- . 
cerned, we have function without medulla- 
tion.”’ 


A further quotation perhaps may be permissi- 
ble because of its bearing on other aspects of the 
problem dealt with in this paper. WATSON finds 
that during the period from the tenth to the thir- 
teenth day after birth in the rat there is present the 
capacity for 


“forming and retaining definite associations. 
The solving of the problems given to the rats at 
the above ages would require the use of the 
olfactory tract (probably at thirteen days the 
auditory tract was also involved), some sec- 
ondary tract to the cortex, the cortex itself, the 
pyramidal tract, and of course the peripheral 
nerves. If we examine the medullation process 


78 H. H. LANE 


at this age, we find that the olfactory tract is 
entirely unmedullated, that a secondary med- 
ullated tract to the cortex does not exist, that 
the cortex is entirely unmedullated, and that 
the pyramidal tract contains but few medul- 
lated fibers.”’ 


He concludes therefore that ‘“‘medullated fibers in 
the cortex of the rat are not a conditio sine qua non 
of the rat’s forming and retaining definite associa- 
tions.”’ 

If then, as WATSON has shown so conclusively, 
medullated fibers are not necessary for the estab- 
lishment of associations in the cortex, and if, as my 
preparations show, a multitude of such non-medul- 
lated fibers are present at birth and many more 
within a few days thereafter in both the brain stem 
and the hemispheres, then it must be granted that 
the central links in the chain of neurones constitut- 
ing the connections between the exteroceptive or- 
gans and the motor mechanism are present, and 
functional at this time, if the presence of neurofi- 
brillae be a safe criterion. Why then is not the rat 
at birth able to see or hear, as well as to feel and to 
maintain his equilibrium? The optic and auditory 
nerves are completely fibrillated long before birth; 
the central connections are probably already estab- 
lished at birth or within a few days thereafter. The 
motor mechanisms that would be involved in the 
response to stimuli of light or sound waves are in 
good working order even before birth. Even the eye 
muscles are differentiating and their innervation es- 
tablished as early as the 16 mm. stage in the rat. 

In short, the chain of neurones from the ex- 
teroceptive organ to the motor mechanism is com- 
plete for sight and hearing, possibly at birth, cer- 
tainly within a few days after, and several days 


CORRELATION OF STRUCTURE AND FUNCTION 79 


before the function is established. The block in the 
circuit is the extero-receptive sense-organ. The rat 
cannot hear before the twelfth or thirteenth day, 
nor see before the sixteenth or seventeenth day 
after birth because it is not until those dates re- 
spectively that the ear and eye have reached a 
functional condition. Looking back over the experi- 
ments and structural observations recorded above 
on the senses of touch, equilibrium, smell and taste, 
it will be found that there too, im each case the 
function is established when and only so soon as the 
proper peripheral sense organ has reached its func- 
tional state. 

_ The course followed in the development of the 
special senses and their correlated mechanisms is 
not just what one would expect on a priori grounds. 
After the earlier differentiation of the neural tube, 
the central connections between sensory and motor 
nerves are established in the cord, at least, and 
probably in the medulla also before or simultan- 
eously with the appearance of such nerves, which 
very soon establish their distal connections. This is 
followed almost immediately by the completion of 
the motor mechanism; then comes the establish- 
ment of the central connections with the higher 
portions of the brain, and last of all the peripheral 
end-organs attain functional capacity. Then, and 
not until then, are stimuli from the outside world 
able to start a reaction that travels from sense- 
organ to central connections and thence out to the 
motor mechanism. That the apparatus as first estab- 
lished is not perfected has been shown above, but 
its later development follows in the paths already 
laid down, and consists probably in the successive 
addition of neurones to the class of those already 
functional. 


80 H. H. LANE 


This order of development is not what is de- 
manded by the Lamarckian hypothesis. If structure 
were to follow from the effects of extrinsic stimuli, 
the logical order would be: peripheral sense organ, 
sensory nerve, central connections, motor nerve, and 
finally, motor end-organ. But such is clearly not 
the case. It follows therefore that the forces which 
bring about the development of the mechanism of 
the special senses and their motor connections, are 
intrinsic; they are forces brought into the organism 
by heredity, that is, they are inherent in the germ- 
plasm. The whole process is due to germinal or- 
ganization though doubtless with enough plasticity 
to allow for a considerable degree of adaptation to 
minor environmental changes, and in a secondary 
sense controlled in a measure by the correlated de- 
velopment of the circulatory, lymphatic, excretory, 
and other systems of the body. Such adaptive modi- 
fications however do not affect the fundamental 
course of development; they concern only its minor 
retails. 


CORRELATION OF STRUCTURE AND FUNCTION 81 


GENERAL SUMMARY. 


1. Fifteen stages in the development of the 
white rat, ranging from 714 mm. embryos to young 
seventeen days after birth, have been examined 
from the standpoints both of structure and of func- 
tion in an attempt at a correlation between the two 
as regards the development of the special senses. 


2. The nature of the neurofibrillae is discussed 
and the theses are supported: (1) that they are ac- 
tual structures of the living neurone; (2) that (a) 
either they are composed of rows of colloidal par- 
ticles held more or less closely together in a linear 
arrangement by means of another constituent of the 
protoplasm differing from them in its degree of 
viscosity, or (b) they consist entirely of a viscid 
substance having the form of strands differing 
chemically and physically from the other elements 
of the surrounding protoplasm; and (3) that they 
are not to be regarded as the so-called “Sttitz- 
gerust’”’ of the neurone. 


3. The function of the neurofibrillae is dis- 
cussed and the conclusion is reached that they 
constitute the conducting elements of the neurone. 


4. Experimental and structural data are pre- 
sented on the sense and apparatus of touch which 
are interpreted as showing: 


(a) That both the sensory and motor nerves, 
as well as the central correlation paths between 
them, are laid down very early in embryonic life— 
were in fact present in the earliest stage studied, 
the 714 mm. embryo. 


(b) That the sense of touch is established 
somewhat later in embryonic life, at or before the 


82 H. H. LANE 


16 mm. stage, upon the development of a tactile 
end-organ; in the first instance this organ has the 
form of a neurofibrillar basket or reticulum in the 
vibrissal follicle. 


(c) That the sense of touch is increased and 
perfected through (1) the addition of new vibrissal 
organs, and (2) through the innervation of the in- 
tegument itself. 


5. Experimental and structural data are pre- 
sented on the sense and apparatus of equilibrium, 
which show: 


(a) That a sense of equilibrium is first appar- 
ent upon the completion of the end-organ con- 
cerned, viz., the sensory cells of the cristae acusticae 
in the ampullae of the semicircular canals. 


(b) That the power of equilibration is gradu- 
ally perfected through increased powers of co- 
ordination with the tactile apparatus (vibrissae), 
and the organs of sight (eyes) as well as the estab- 
lishment of the general muscle tonus. 


6. Experimental and structural data are given 
for the sense and apparatus of smell. It is shown: 


(a) That no certain response to odors was 
made until the olfactory epithelium contains fully 
differentiated olfactory sense-cells. 


(b) That the olfactory tracts in the brain de- 
velop previous to and independent of the peripheral 
organ of smell. 


7. Experimental and observational data on 
the sense and apparatus of taste are set forth, 
which show: 


(a) That the trigeminal and glossopharyn- 
geal nerves are both concerned with taste. 


CORRELATION OF STRUCTURE AND FUNCTION 83 


(b) That both these nerves and their central 
connections are completed long before birth and 
long before a sense of taste is present. 


(c) That taste comes some time after birth 
upon the development (1) of peculiar gustatory 
organs on the anterior part of the tongue, and (2) 
of taste-buds on and around the circumvallate pa- 
pilla. 

8. Experimental and observational data on 
the sense and apparatus of hearing are presented, 
that indicate: 


(a) The early establishment of the auditory 
(cochlear) nerve and its central connections. 


(b) The late development of the organ of 
Corti. 

(c) The dependence of the sense of hearing 
upon the establishment of the definitive structural 
conditions in the end-organ of hearing (organ of 
Corti). 

9. Experimental and observational data on 
the sense and apparatus of sight are given which in- 
dicate: 


(a) The early establishment of the optic nerve 
and its central connections. 


(b) The very late differentiation of the reti- 
nal elements and the accessory structures of the eye 
in general. 

(c) The fact that sight is not possible until 
the whole apparatus is in working order, of which 
the last element to be perfected is the sensory end- 
organ. 

10. The causes of development and of differ- 
entation in the nervous system are discussed and 
the conclusions reached: 


84 H. H. LANE 


(a) That the development of the nervous sys- 
tem in general, and the differentiation of its con- 
stituent parts, are the products of endogenesis, or 
predetermination in the odsperm, and not of epi- 
genesis. 


(b) That these structures appear not as direct 
responses to the needs of the embryo, but in an- 
ticipation of those needs; not under the influence 
of their specific, definitive environmental stimuli, 
but because of the inherited organization and forces 
in the odsperm. 


(c) That the early establishment of the peri- 
pheral connections on the part of the nervous 
system receives its explanation in the mechanical 
conditions existing at an early stage in embryonic 
development but not later, viz., that it is only 
in these early stages that the distance between the 
neural tube and the surface of the embryo is within 
the limit of independent growth of the neuronal 
processes. 


11. The fact is shown that in the case of each 
sense the chain of neurones from the exteroceptive 
organ to the motor mechanism is completed and the 
sensory function established only when the proper 
peripheral sense-organ has reached its functional 
state. 


12. It is pointed out that this unexpected order 
of development is contrary to any hypothesis of ex- 
trinsic causes, and that the forces concerned in the 
development of the mechanism of the special senses 
and their motor associations are those inherent in 
the organization of the germ-plasm, and may be 
only secondarily modified or controlled by other 
factors. 


CORRELATION OF STRUCTURE AND FUNCTION 85 


HPILOGUE. 


The author is fully aware that this paper is 
only of a preliminary nature, outlining the field and 
establishing a few land-marks or base-lines for fur- 
ther investigation. As fast as the material can be 
obtained the relatively wide gaps in the above- 
given account of the rat will be filled in, and in 
addition comparative studies on other mammals are 
already planned and partially under way. Thanks 
are due to the National Academy of Sciences for a 
grant of $500.00 with which has already been pur- 
chased the equipment necessary to continue this 
work. Circumstances render it desirable, however, 
and the results so far obtained seem sufficiently . 
important to warrant their early publication rather 
than to await the more detailed account that can 
come only after the expenditure of a much greater 
amount of time and labor than has been so far 
available for this study. 


86 


H. H. LANE 


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